The difference is in motorsports, roost is a result of going fast. Fast is cool. On mountain bikes, roost is usually a result of sliding while cornering - which is slow. Plus, it wrecks the trail surface. Not cool.
Have you ever seen a fast rider hit a corner? *Roost!*
I agree with you when it comes to skidding photos, as that is pretty lame, however the existence of roost doesn't mean a rider is going slow - if anything it usually means the opposite.
Everyone providing valid arguments. But in my opinion watching an athlete charge down a mountain and carve side to side (similar to a skier/snowboarder in deep pow) is simply beautiful.