Mercs were pretty poor with normal slotting, but they really benefited from the Achilles Heel IO proc. By putting into the first two tiers, you now have 5 sources of Achilles Heel debuff, which really increased their damage.
Granted, having to use a proc to give the set some specialty that it's actually good at, is poor design that just happened to luck into a decent combination almost by mistake. The base powers for the minions were lackluster... no real high damage, no huge -Regen like the bots, no great control and self-heals like Necros, etc.
Oh, wait... they did have one minor thing. You could set up the Spec Ops, stealthed (or with a Smoke Grenade) to go next to a Boss, and then trigger them to attack. If they were in melee range, their first attack would be a butt-smash with their rifles, which stacked up to a Boss-level stun to start every fight. But they didn't really have enough control ability to make it worth slotting for control even with Hami-Os, so the stuns were pretty short duration... 5 seconds or so.