Personally I prefer a MC Fighter/Thief to the *pure* Thief. Reason being, pure Thieves never really get a good THAC0. So, the later skills, such as Assasination, Gr. Whirlwind Attack and even the Time Stop Traps are almost useless to a pure Thief as they have a very hard time being able to hit the low AC enemies.
Now, a MC Halfling Fighter/Thief does not have this problem. His Fighter portion just keeps getting better and better. So, now with this low (-3 to -5) THAC0, he can make excellent use out of the aforementioned high level skills.
Oh, and have I mentioned that Halflings get excellent saving throws? They are excellent fighters as well. My level 23 MC Halfling Fighter/Thief can kick the crap outta my level 18 Inquisitor. Of course he is weilding Celestial Fury, but that's another discussion.
Also, with being a Fighter/Thief they have many more options as far as armor and weapon selection. They can wear helmets (which pure Thieves cannot). They can use shields larger than the crappy buckler that pure Thieves are stuck with. And they can wear heavier armor than studded leather (though this does disable their stealth ability, but then we do get a ton of invisibility potions
) which can get you a very low AC. I've been able to get -14 AC without really trying, with a Fighter Thief.
So, bottom line is a *pure* thief is good for looting and scouting, but if you want your "Hero" to be able to mix it up, get a MC Fighter/Thief. I suggest Halfling for the saving throw bonus.
BTW: I have tried all the Thief kits and pure thief a few times before I cam to this conclusion.
Hope it helps.
-Immoral