Mercs in D2 were a crutch. Plain and simple. But if they do put them in D3 they should put some disadvantages into them. For example, you gain zero (or reduced) exp and/or drops from your merc killing (or doing the most damage) to a mob.
The only other option would be to make mercs so nerfed that they couldn't kill anything by itself and/or would be squisher than a rotten tomato.
After all that said, I understand why some love to play dress up with mercs but it shouldn't be necessary to play gear master of Sanctuary to be viable in the game.