Two of the guys in the beer-inspired conversation were on the big side of 90kg/200lb (as am i), so there was a certain extra element there. We all also ride in hilly areas. One guy has ridden disk equipped CX bikes before and thought it a huge improvement. I suppose the main usage would be for a bike suited to audax/sportif events.

Mounting locations - as Unterhausen mentioned.
I guess that now that I think about it most MTB put them up on the top of the seat stay. The stays are thicker and heavier (and shorter) than a road bike.
Many of the CX and commuter bikes I've seen photos of seem to have them inside the triangle on a bracket that connects the chain stay and seat stay, presumably acting as a significant brace as well as putting the mount in the right place.