I would say always use a merc, even when they no longer help combat wise. Just drag them around solely for the xp boost. Thats what I did last time I played, and I was doing a thief. Find a low damage high xp NPC, haave the giant rescue, and start using circle. Worked easily enough. I think the Giant from Galadon is neutral so goodies can use that one.

If you are a class that can buff the merc, all the better. Transmuters and bards do well, healer less so at later ranks just cause you suffer from minimal damage output. I could solo rank a bard using a merc in 50 hours pretty easily.Anthem, battaglia and frenzy talismans go a long way in making the merc more useful. I seem to remember healing songs not healing mercs though, but I don't actually remember.