There is now research that shows that unabsorbed carbohydrates may lead to problems with digestion and to an increase in bacteria. Even when normal subjects ingested difficult to digest carbohydrates (lactulose and polyethylene glycol), they developed signs of GI disorders.
It is the structure of the carbohydrates that determines the difficulty for absorption.
1. There are three types of carbohydrates: simple sugars (monosaccharides), disaccharides (such as lactose and sucrose), and polysaccharides (starches).
2. During digestion, the body must break down all carbohydrates into monosaccharides before they can be absorbed. Monosaccharides are the only carbohydrates that can be directly absorbed by the body without being broken down.. The disaccharides are too large to penetrate the cell membrane.  Polysaccharides are larger than disaccharides so they also need to be broken down before they can be absorbed.
3. Some people have carbohydrate malabsorption and are unable to breakdown disaccharides and polysaccharides. If the body cannot break down the carbohydrates, the intestinal bacteria will have access to plenty of food  and will overgrow.
4. An overgrowth of pathogenic bacteria can lead to enzyme deficiency and mucosal damage in the GI tract and other GI damage.
5. Several research experiments found that digestive problems will resolve if patients consume only monosaccharides and avoid the other types of carbohydrates. The SCD advocates this approach; monosaccharides are the predominant carbohydrate in that diet.
Conclusion: People who have carbohydrate malabsorption should avoid eating disaccharides and polysaccharides in order to avoid bacterial overgrowth and digestive problems and allow the microvilli to regenerate.
A more detailed, comprehensive and scholarly explanation of this topic is provided in the book, Breaking the Vicious Cycle, using other references.
(Website of the Ohio State University at Mansfield.)
 Proof that carbohydrates need to be broken down into monosaccharides in order to be absorbed:
"Monosaccharides can be absorbed directly into the bloodstream, but disaccharides need to be broken down into their monosaccharide components before they can be absorbed."
(Website of the Ohio State University at Mansfield.)
"Among carbohydrates, monosaccharides and, especially, glucose tend to be much more available for use (e.g., for energy) than either disaccharides or polysaccharides. This is because carbohydrates ultimately are avalable as nutrients only once they have been broken down, by specific enzymes, into a monomeric form."
Caspary WF. Carbohydrate absorption and malabsorption, Leber Magen Darm. 1977 Jun;7(3):150-9. http://www.ncbi.nlm.nih.gov/pubmed/578285
"Starch is digested intraluminally by alpha-amylase to maltose, maltotriose, and alpha-limit dextrins. These products, as well as the disaccharides sucrose and lactose, undergo enzymatic hydrolysis to monosaccharides at the brush border surface. The monosaccharides enter the absorbing cell by specific transport mechanisms ("carriers"). Primary carbohydrate (CH) intolerance is characterized by the congenital or acquired absence of individual brush border enzymes or of monosaccharide "carriers" without morphologic abnormalities of the intestinal villus: lactose, sucrose and trehalose intolerance and glucose-galactose malabsorption (brush border diseases). Secondary CH intolerance arises when surface digestion and absorption are reduced due to structural changes of the intestinal mucosa: e.g., decrease or absence of villi with sprue and reduction of the absorbing surface with intestinal resection. Watery diarrhea is the lead symptom."
Proof that disaccharides are too large to penetrate the cell membrane
Herber, R. Disaccharidase Deficiency in Health and Disease. Calif Med. 1972 June; 116(6): 23–37. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1518335/pdf/califmed00126-0057.pdf
page 25 of journal (page 3 of pdf):
"Ugolev, however has evidence for a sucrase enzyme on the outside of a membrane. This concept is also in keeping with the finding that the molecular diameter of the disaccharide is too large to penetrate the cell membrane."
Page 24 of journal (page 2 of pdf):
Intact disaccharide appears incapable of quantitatively entering the absorptive cell from either the intestinal or vascular side.
Levin RJ. Digestion and absorption of carbohydrates--from molecules and membranes to humans. Am J Clin Nutr. 1994 Mar;59(3 Suppl):690S-698S. www.ajcn.org/cgi/reprint/59/3/690S.pdf
Colonic bacteria obtain their food supply from carbohydrates that are not absorbed.
Sylvia H. Duncan,1 Karen P. Scott,1* Alan G. Ramsay,1 Hermie J. M. Harmsen,2 Gjalt W. Welling,2 Colin S. Stewart,1 and Harry J. Flint1. Effects of Alternative Dietary Substrates on Competition between Human Colonic Bacteria in an Anaerobic Fermentor System.
Appl Environ Microbiol. 2003 February; 69(2): 1136–1142.
doi: 10.1128/AEM.69.2.1136-1142.2003. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC143671/
"Colonic bacteria obtain energy by fermenting a variety of dietary compounds that are not digested by the host" (First paragraph after the abstract)
Grimble G, Fibre, fermentation, flora, and flatus.
Gut 1989;30:6-13 doi:10.1136/gut.30.1.6
 Proof that there is bacterial growth when there is an increase in unabsorbed carbohydrates.
Shetty PS, Kurpad. AV. Increasing starch intake in the human diet increases fecal balking. Am i Gin. Nutr. 1985;43:210-2. http://www.ncbi.nlm.nih.gov/pubmed/3004188
Bacterial overgrowth damages the digestive system
Mathias JR, Clench MH. Review: pathophysiology of diarrhea caused by bacterial overgrowth of the small intestine.
Am J Med Sci. 1985 Jun;289(6):243-8. http://www.ncbi.nlm.nih.gov/pubmed/3890541
"The bacterial overgrowth syndrome constitutes an intestinal problem involving alterations in motility and injury to the brush border and mucosa. The overgrowth of bacteria also causes secretion, malabsorption, and maldigestion. These alterations result in a clinical syndrome that manifests itself as weight loss, malabsorption of specific nutrients, and (usually) diarrhea."
Monosaccharides are well tolerated by people with digestive disorders
BOWIE MD, BRINKMAN GL, HANSEN JD.
Acquired disaccharide intolerance in malnutrition.
J Pediatr. 1965 Jun;66:1083-91. http://www.jpeds.com/article/S0022-3476(65)80095-6/abstract
"The possibility that an acquired disaccharide intolerance is in part responsible for the diarrhea of protein-calorie malnutrition has been investigated. A malabsorption syndrome is a well-documented entity in malnutrition but, in addition, 60 per cent of the children in this series had evidence of a fermentative diarrhea. This was improved by the removal of carbohydrate from the diet. Monosaccharides were then efficiently absorbed but not lactose. Gastrointestinal infection and infestation appear to play a role in producing disaccharide intolerance in the already damaged gut of protein deficiency."
Lifshitz F, Coello-Ramirez P, Contreras-Gutierrez ML. The response of infants to carbohydrate oral loads after recovery from diarrhea. J Pediatr. 1971 Oct;79(4):612-7.
" A clinical study was carried out to assess the response of 46 infants to oral loads of carbohydrates after recovery from severe diarrhea. The response was measured by the increases in blood reducing sugars and by the variations in the stool pattern following administration of oral carbohydrates. Disaccharide oral loads were used in the test. During the acute stage of the illness, 38 of the 46 patients had exhibited intolerance to carbohydrates, as evidenced by the excretion of acid stools and/or stools with a greater than .25% carbohydrate content. Prompt improvement from diarrhea was induced by elimination of all lactose, disaccharides, and other carbohydrates from the diet. None of the infants had monosaccharide intolerance during the acute diarrheal stage."
H. A. WEIJERS 1 , J. H. VAN DE KAMER 1 , 1 J. IJSSELING 1
Diarrhoea Caused by Deficiency of Sugar Splitting Enzymes. Acta Paediatr. 1961 Jan;50:55-71.
In 3 children suffering from chronic diarrhoea in which all normally occurring causes (including coeliac disease) were ruled out, the absence (or defective functioning) of invertase and/or maltase was proved to be the cause. The diarrhoea was arrested with a monosaccharide diet, and in the case of invertase deficiency also an invertase preparation helped.
Both secondary and primary (genetic) disaccharidase deficiencies are known, which may lead to malabsorption and intolerance of the corresponding disaccharide(s) but not of the constituent monosaccharides. Diagnosis depends on determination of enzyme activities through small-intestinal biopsies and/or on an oral tolerance test with the corresponding disaccharides. Among the latter, the breath hydrogen test is particularly reliable.
H F Hammer, K D Fine, C A Santa Ana, J L Porter, L R Schiller, and J S Fordtran.
Carbohydrate malabsorption. Its measurement and its contribution to diarrhea.
J Clin Invest. 1990 Dec;86(6):1936-44.
H F Hammer. Colonic hydrogen absorption: quantification of its effect on hydrogen accumulation caused by bacterial fermentation of carbohydrates.
Gut. 1993 Jun;34(6):818-22. http://www.ncbi.nlm.nih.gov/pubmed/8314516
Hammer HF, Santa Ana CA, Schiller LR, Fordtran JS. Studies of osmotic diarrhea induced in normal subjects by ingestion of polyethylene glycol and lactulose.
J Clin Invest. 1989 Oct;84(4):1056-62. http://www.ncbi.nlm.nih.gov/pubmed/2794043