Tank mercs are overpowered at lower levels, so even melee characters hire them to be effectively powerleveled for the first 60-70 levels. At those levels tank mercs do just fine without heals, at least in older zones. Then, when the tank merc starts falling behind, you can switch to a healer merc.
Personally, I don't like this approach since it feels like I am not playing my class and I don't feel character improvement from accumulating gear, levels and AAs, but to each his own.
P.S. If your screen name indicates your primary class and you are a Necro, keep in mind that SKs have fairly low DPS at first. It gets better as you improve your character, but at first it feels painfully slow after playing a DPS class. Make sure to stock up on clarity and haste potions :)
Edited, Jan 13th 2010 12:36am by Muon