The adage of getting more air with wider bars is common sense, but we're talking pretty much shoulder width being the point of gain. Think about it though: on a road bike you're pretty much in one position whereas on a mtn. bike you're moving so much anyway.

If I understand trends at all, the new "wider is better" is coming from the 29er movement where more leverage is supposedly needed to get the big front wheel to move for turning purposes. True of not, I like 'em wide which comes from my old motorcycle dirt bikes from the 70's and we always had wide bars. (Yeah, and I have a 9er...) It is all a personal preference and that's why there are so many options available..