I can't dig up the thread right now, but it was mentioned, for one, by Pat Rogers that he didn't like to see pistol lights run on carbines in his classes. I didn't see reasons given.
I have a picture of Pat running an X300 at 12:00 in front of his FSB. Maybe he changed his mind.
The "pistol" lights are light and compact, and rugged enough (surefire X300 or streamlight TLR-1).



















































