I can't speak for PVP, since I rarely PVP. But my main is a Merc Healer, and is comparitively geared with another sorc healer in my guild. We routinely run EC SM, and I hate to say it...but my heals suck compared to his.
Are Merc heals useless? No, there are benefits to some of the abilities we have. Kolto Missle combined witht he Sorc AOE HoT can heal up much faster. The damage reduction buff we can put on people helps in times of massive dmg going out. But when it comes to just straight healing, trying to keep a tank alive or the random DPS'er that accidently pulled aggro...I just can't keep up with the sorc. I love playing my merc, but I am almost done leveling a Sorc healer so I can swap out...because the Merc just isn't cutting it any more.
Heat can be an issue, but I have become very good at managing it and rarely overheat during most fights...but if i have to start throwing out my biggest heals...its over before it even gets started, while the sorc is still rolling right along making me look like a fool.
All this being said, We still make it through...so it's doable just challenging...which I personally don't mind.
--EDIT: After reading a few posts before mine, I wanted to come back and add that single target heals is actually a merc strong suit in my oppinion. If all i had to do was heal the tank/s I would be rocking and rolling. Sadly, as with most raids, that is not the case for me as damage is never just focused solely on the tank. Now, if i could just find perfect DPS'ers that never took damage...I would be unstopable! but what fun would that be.
