In my opinion, mercenaries are NO Replacement for a real player. They are good, when you need a rez, clr buff, or a healer that is attentive but not so bright, and generally just casts hots, promised heals, and complete heals, rarely casting quick heals, depending on stance, and are at their best in an efficient stance for grinding easy content, you could solo, but would have some down time every few mobs for health / mana.
At least the clerics med, when they can, but they are very inefficient even in efficient stance, and not good at judging situations and adjusting. Warriors, never med, and run out of Endurance fast in any group. As others have said, clerics have huge heal aggro, and their over healing doesn't help. You can work around alot of their problems, but it takes different strategies and makes the group way less efficient overall.
And I'm not sure, but from observation, it seems they take more than their far share of experience. In every circumstance I've been in, replacing a merc with even a non-active player ends up with more experience, unless it was the only healer in the group, and resulted in lots of downtime without the cleric merc.
One of the best things about mercs tho, is they do pay attention and react even if it's not the best of all reaction choices. Another descent use, is a slaughtering group taking low damage, you can suspend the merc for 5mins then unspend long enough for a complete heal, then re-suspend and continue grinding.
Generally I've only found a good use for War mercs for nameds or other multiple mobs, in a group without crowd control, as they're descent off-tanks, but again will prob burn up 50% of their End during the fight, and require forever to regen it, since they never sit.
So, overall, they have their uses, but are in no way a replacement for a real player that is at least moderately attentive, and knows the very minimum of how to play their class.
Yther Ore.