I do believe in visceral hypersensitivity...why not? The GI system contains as many discrete neurons as the CNS, so why couldn't the receptor feedback loop fall into a tonically "excited" state.
I think trying to label all visceral hypersensitivity as IBS is part of the problem...whereas, if you think of it on a spectrum ranging from occasional episodic abdominal pain to chronic alternating diarrhea/constipation and pelvic floor dysfunction it becomes easier to understand. At least IMO.
Me thinks you just haven't had the concept presented to you in a logical manner. Citrucel and go in peace.