personnaly having done recreative MTB and turned to competitive 'cross I have so much fun in trails when they are not very technical. ortherwise you get a bit beaten up and end up having your bike on the shoulder running! When I ride with friends whom have MTBs they are always surprised how easily I climb any hill and if the gradient rises I jump off the bike, shoulder it and wait for them up the hill!

as for the purist point of view, I think all bikes have their purpose but think that some ppl end up with bikes a bit too much(i mean full sus. bikes for light XC). And I let you wonder how cool it is to pass some guys in MTB and they say ''man , wath are you doing here on a road bike!?'' then you reply ''its not exactly a road bike'' and leave them behind! 8-D