One of the main reasons that mountain bikes are so popular is that they are so doggoned versatile... truly a go-anywhere bike, on-road or off. Plus, the generally upright position makes new or infrequent riders a lot more comfortable. And since it's the bike you already have and are comfortable with, it's the best bike to ride.

It's probably not the optimal bike to ride for a road time trial (which is what the bike phase of a du- or triathlon really is) due to the big knobby tires, off-road gearing, and overall weight, especially if you're looking to win the thing. Obviously that's going to depend on your engine...