If you only have one bike that you are going to ride on the road, it should probably be a road bike, not a tri-specific bike. I think it's nice to have both kinds- road and mountain. Even if you just ride easy trails, you can't do that with a road bike, and the mountain bike _can_ go on the road, but things will be so much easier and more fun if you have a skinny tire bike.

You do see mountain bikes at triathlons, but infrequently. But if that's what you have, and you want to do the tri- go for it!!

Nanci