I am fine with 31.8 if it means better cross-compatablity between road and mtb bars.

I find 31.8 on drop bars to be of no real benefit for me, and find the extra bulge makes it a little harder to utilized some of the bar around the stem.

However, on MTBs I think they make a lot of sense, in light of bars being up to 800mm wide (mine are 750mm) and the amount I lean into and pull on them.

So while would slightly prefer a narrower clamp size for road bikes and drop bars, I am willing to accept the 31.8 standard if it means that stems are interchangable between mtb and road.