a lot of the guys in the club/team actually do ride both... when we were voting for officers a few weeks ago, it came up that one of the guys running for prez. didn't own a mtn bike, and perhaps he wasn't as dedicated to the club as the other candidate? he assured us that he was planning on buying one, and he was elected prez.
maybe the difference is the collegiate "seasons"... if you road ride, you're not going to get to race August-November, but if you just mtn, you'll be sitting spring out. about 1/2 of the team dabbles in both equally, but the majority of group rides are road, simply because that's what we have around here. to get to any decent, marked singletrack (i say marked because there is a local loop that you can go get completely lost on) you have to drive about an hour.