Yes, thieves w/o steal would be weird. And I agree a thief should not be able to stand toe to toe with a warrior melee-wise.

You point out that they are fragile and rely on stealth. Yes, but assassins can stealth better than a thief and are not fragile. They can hide from a thief and can attack with trance dam redux and +200-300hp? with an edge.

Also, consider thief paths. A lot of times it feels like people think of thieves and what they can do without considering the trade-offs with other skills and that a thief with skill A will not have skill B and vice versa.

If assassins were broken up like thieves, they might go down the throw path and not get trance. Or, if they go the trance path, they won't get kicks, etc.

Frankly, I'm not sure that a very reliable KO skill is overpowered when it lasts an hour, people can wake up from a failed steal (cabal power maybe?) and when you attack them, most can kick your ass or get away if they've quaffed a 150 copper potion.