I have a mountain bike, just a bottom of the line GT hardtail. It gets used as a back up bike, round town, and on the cycleops since I don't want to mess up the lovely Record on the road bike.

I can see that any off road riding helps my bike handleing skills but I don't bike ride because I'm disgustingly practical. Besides pure pleasure of riding I ride to get from point a to point b, sometimes point a to a if it's a loop but as an alternate to driving.

Also I hate the sight of blood, especially my own, and while with my other sport (Aikido) I take falls for fun, no actual blood is ever involved with that. And like Spazz was saying while people (self included) can be surprisingly uncoordinated although we're athletes...I'm sure it'd be carnage if i went off road.