I look at it as a tradeoff. Do you want max TH or max DPS?
TK is a weapon. That means that if you have a TK "replacement", the result will be people demanding you wield the replacement + Thief's Knife to get more shinies instead. You can't win.
I'd laugh my ass off if we see a TH+2 weapon in the next expansion....and it's THF only, Hand-to-hand. Call em "Golden Gloves" or something. At that point, we can look at S-E and know the true purpose of THF is TH whoring. ^ ^