I bought a MTB in 99 becuase I couldn't find anyone to ride on the road with. I would go out with my buddies who considered themselve pretty hard core and kill them on the climbs then they would kill me on the decents. I find even "difficult" climbs pretty easy compared to what I consider a difficult road climb. The decents are pretty tricky do to the fact that the trail is never predictable but other that that I find MTB to pretty boring.

The other issue is that MTBing seems to mean loading your bikes up and going somewhere to ride as opposed to leaving your house on your bike. This aspect of road riding is one of the best parts, I rarely actually put my bike on the car unless I'm running late for a team training ride.