I'm fairly leery on mercs myself, especially having used them extensively in EQ1. I can see how they'd be helpful for some classes that are weaker or even tiny guilds wanting to try heroic content but overall I think the game would've been fine without them.
However, the detail the panel went into on them sounded pretty neat. They're aiming for each race to basically behave differently: examples given were maybe a Dark Elf healer would be more of a battlepriest and a Halfling healer would possibly run in terror when there's a few too many mobs. It sounds like their city of origin might also affect behavior.
So like I said, personally I'm iffy on them, but we'll see. If anything it should get some money sucked out of the economy.
True, they were quite handy in EQI I have to admit. But in that game, it's quite hard to level on your own, and with not that many people playing or at least levelling new characters I had to use a healer merc, but it's never better than the real thing.
But that's just never a problem in EQII imo. Sure, a soloing player will not run tough instances or quests when they're green or better, at least not levelling up; but with mentoring and the chronomages, that's not that hard anymore either.
I'm not sure they'll create merc healers or tanks that will be able to replace actual people in raids or tough instances. And for soloing, I really can't see the need for them.