There are ways to cheat to the top of the healing meters, so the only way I find healing meters accurate is if nobody is cheating. Cheating can really cause problems in raids.
An example are the single pull Arcane Protectors between the Curator and the Shade of Aran in Karazhan. The most efficient healing method with these pulls would be for all of the healers to cast flash heals on the main tank non-stop, even when the tank is at 100% health. Often times, people in my guild attempt to cheat their way out of stacking up on the overheal meters by dancing big heals or only casting flash heals while the tank is missing health. Considering how hard these Arcane Protectors can hit once they go enraged, and the frequency of their hits, the tank could be dead in 2.5 seconds, which can be a problem if the heals aren't staggered, which they usually aren't considering all the healers start their casts once the tank is missing health, meaning they'll all finish at the same time.
That is just part of the argument I could have, for just that one mob. Healing Meters can cause huge problems for raiding guilds, yet can be a fun tool to allow that allows further progress so long that everyone is healing efficiently.