If they survive the fight, they are good to go again without much delay.

I'd agree with your idea of always having a merc, except that my issue is normally that I level up too fast for my skills/spells/supps/songs. Only time I really feel the need to use a merc is if I found some sweet item of limited gear on a sub-level 11 character, and need to hit 11 before a crash or being forced to log off.

But yeah, if you know where low dam high exp mobs are, you can rank nicely with a merc.

I've seen people using the giant to kill the Juju zombie outside the maus, even.