Okay, so a lot of Rebel pilots were originally Imps: examples are Biggs (defected after the Academy, near Bestine, as said in the deleted scenes), Han Solo was the top TIE pilot (beat Baron Soontir Fel) in his class, Kyle Katarn was the top Stormtrooper in his class, and Tycho Celchu and Derek "Hobbie"Klivian were both also Imperial pilots.
Chewie stole an AT-ST just fine.
As for gameplay reasons, I know some people enjoy being the ruthless Imperials, but also like to control fighters such as the A-Wing. However, if the vehicles aren't locked, stealing them should be difficult. A soldier would have to infiltrate the enemy base, without being killed, get the starfighter or other vehicle, and escape without being killed.