I'm on record as a certified "hater" of the class, because I think it steps on the toes of too many other classes and because I think Thief abilities open the door to a more generalized skill system, which I see as a large nail in the coffin of old school play. For the past few weeks, I've been trying to re-evaluate my stance to determine if I'm being reasonable. A big part of me thinks I should just cut the poor Thief some slack, since the class has been around since 1975 -- long enough to have entered into the traditional canon of the game and thus become, by my usual criteria, sacrosanct. After all, if 4e had cut the Thief/Rogue from the game, I'd be screaming bloody murder about it, wouldn't I?
Actually, no. The more I think about it, the more it becomes clear to me that the Thief is a self-justifying class. Prior to its introduction in Greyhawk, pretty much every "thief ability" was something a character of any class might attempt. Listening at doors? Check. Moving silently? Check. Locating and disarming traps? Check. The list goes on. I've noted before that "thief" is an occupation that could describe characters of any class. If you consider the pulp fantasy antecedents of OD&D, you soon realize that many, if not most, of the protagonists of such tales could be rightly called "thieves" even if none is at all similar to the Thief class. That suggests to me that being a thief is more a question of what you do as opposed to who you are -- and "who you are" is exactly what a character class represents (which is why I object strenuously to the conflation of class with profession).
What's also interesting is that, as D&D has developed, the Thief class has moved more and more away from being the dungeon trap-removal specialist he was in OD&D and AD&D. He's now, for wont of a better word, a ninja -- the stealthy, high damage-dealing class. Now, I think there's a place for such a class in even OD&D, but we already have one and he's called the Assassin. It seems to me that, as the years have worn on, the Thief/Rogue has become more and more like the Assassin, to the point where, under 3e, for example, the class doesn't necessarily have to be good at opening locks or picking pockets but it's inherently good at dealing lots of damage through subterfuge.
So, my plan at present is to treat "thief" as an avocation for other classes and use some version of the Assassin (which I'd dearly love to call a Slayer in homage to Lieber) as the "ninja" archetype class. I think eliminating the Thief might encourage players of other classes to try to be sneakier and I also think that, without the Thief, the temptation to introduce a skill system on top of D&D's class system will be greatly lessened -- a good thing indeed!