The response's to this post are amazing. The motor doesn't care were it's being ridden, as long as it's receiving the correct amount of air-fuel and lubrication it'll run/last forever.
I own a 300exc and dual sport it. I ride the Sheet Iron 300 on it. 20-30 mile sections of pavement and dirt roads with single track. The bike runs perfect.
If the bike is jetted AND GEARED correctly for the riding your gonna do, it will perform perfectly.
Ever here of a KDX200? It's sold as a street bike in Japan. Those guys regularly get 15K miles from a top end and 25-30K miles from a bottom end. Come on guys... this isn't 1967.
To the guy that said he never re-jets his 2 strokes and leaves them stock... be careful. If you go to Colorado or my neck of the woods, we've got some "Thin Air" up here!