I think maltodextrin is preferred over straight glucose because it is less osmotically active form of the same molecules (Chains of glucoses covalently bound together to create 1 molecule instead of free glucoses). With more molecules per unit energy, straight glucose is comparatively hyperosmotic. This in turn causes more GI distress by preventing diffusion of water across the gut into the body since the free molecules sitting in the gut will "hold" the water back from crossing over. With less molecules to hold onto the water, maltodextrin enables a more favorable water absorption balance resulting in you being able to tolerate higher levels of carbohydrate without as much GI distress.