Which version of the game are you playing? I've never seen a Thief failing to escape battles on NES, unless it's a fight with no escape (bosses, some certain enemies). He still escapes quite often on WSC, although he can fail, and I think it's a bit like that on PSX too. I don't know about DoS, but the Thief can hit the enemies there so many times that it's already a good reason to have him in the party.