Not only is this guy a [censored] but he is completely useless in the quest you team up with him,no wonder the thief guild is in such a bad way.This guy can not even finish off monsters that have a slither of health left!!

... Keep playing, You'll see.

lol. Yeah keep finish that quest...

OT: he wasn't that bad of a sneaker compared to some other companions

Mercer is stuck at the level you entered the guild at, the same as every other NPC or follower is locked to the level you first meet them at. Thats why he svcks so bad.

I had lots of fun with Mercer during that quest... Sneaking on ahead, waking every Draugr, then standing back and watching him get clobbered...

The guy's an a$$ and deserves to die, for several reasons (which you will find out as you progress further along in the TG quest line)