For me it very much depends on whether the target is otherwise engaged in melee. If yes, it's pretty easy for a Thief to disengage, hide for a round, then strike again. And I've always had it that a caster is pretty much unaware of anything while in process of casting, meaning they're easy pickin's if the Thief can get close enough..
All through 1e and even in 3e we've played such things as "total then multiply", meaning any bonuses get multiplied as well; and while you don't often get a straight kill on a backstrike at higher levels you can sure soften something up.
The Thief was almost always the party's scout and trap-finder and ideally the DM would reward such activities with some xp now and then. That said, I've always suspected the reason for Thieves needing fewer xp to advance is that the expectation was that they wouldn't get into combat unless they had to, and thus would get less xp than most other classes.