The mercs are good.
Higher mercs cost a bit more but are more than worth it.
They count as a group member so you can use them to get missions and to make leader AA's work, the merc also takes exp like a real group member.
As you level so does your merc.
Never wait for a rez or cleric buffs again, and they don't afk.
For a mage, it takes the worry of healing your pet out of the game so you can just concentrate on DPS, thereby allowing you to do higher content without waiting to join a group.