I'd never really thought of a saddle as being specific to one kind of bike over another. It's an AR5£-frame interface (tm) and I suspect that MTBers' botties are similar to road botties, though opinions might be split! (I own both kinds of bike and I'm sure my bum can't tell which bike it's on without the aid of my eyes). Although on the mountain bike I don't seem to spend that much time actually in the saddle, and I usually find that saddles seem to be uncomfortable or comfortable more as a function of how often you subject your butt to them than differences in design. By which I mean I just sit on it and accept it – not that I am trying to say there aren't useful differences. I haven't ridden more than 50 miles in one go for a while though, so for people that ride double that distance I could imagine them trying out quite a few until they find one that suits.
Then again, I have one pair of bike shoes and I have Crank Bros Candy pedals on all my bikes for minimum hassle. I have received some mild stick from the more image-conscious of roadies that I ride with, but I just say that it saves me buying another pair of shoes, and I don't have to walk like an incubating penguin when we stop at the cafe! ;-)