I build them as many as I can, but not with the idea to bash some opponents; that squeeky noise of a chariot getting crushed is painful on the ears, you know.
It's about upgrading for me, although they do have some other uses. Scouting has already been mentioned, but also the micro-management of towns becomes a bit easier with them - oops, that town is about to riot, no MP... oh wait, I can get that chariot over just in time!