I imagine most commuters would prefer road bike on the road in a lot of cases (I do), but when I'm commuting or running errands, I never know what I may face. One day my usual route on the MUP led me to a section that was flooded. I would never have expected this in this particular spot, and I wouldn't have wanted to subject any of my "nice" road bikes to 12" deep water. Yes, they could have gone through it, but in that case I was glad to be on my old mtb that I don't mind abusing. And I love riding my Nishiki, Univega, and Bertoni, but there's no way I'm locking them up at Kroger while I go inside and shop. I don't want anything to happen to them. Plus only the Univega could carry much of anything anyway. For me it comes down to cargo capacity and which bikes I'm willing to put through hell and which ones I want to go easy on. It also depends on what bikes a person owns. If I were building up a bike for commuting I'd probably go with something like a Surly LHT which of course is a road (touring) bike. But bikes like that are designed for rough stuff, and so are mountain bikes.