A week ago greedy goblin Gevlon posted why he thought that healing meters are important to measure a healer's performance, to violent disagreement of the healers in his comment section. Of course he was talking about an ideal healing meter, which would take all of a healers actions into account, including things like decursing or shielding. Unfortunately such a healing meter doesn't exist, and there are good arguments why it isn't even technically possible to build one. The real existing healing meters can be gamed, which is the subject of a famous post on how to top the healing meter; that post is extremely funny for a real healer, because it shows how by maximizing your position on the healing meter, you are actually minimizing your real worth as a healer. The worse you heal, the higher you score.
Nevertheless healing meters are a problem, because they exist. People look at them, and judge you according to them, often not knowing how little they actually tell you. Thus when I let Recount run for a whole 25-man raid with 7 healers (2 druids, 2 paladins, and 3 priests) and the three priests ended up on places 5, 6, and 7, I was't happy. How can a class specialized in healing end up bottom of a healing meter, behind the druids and paladins?
The answer is easy: The positions on the healing meter not only depend on the performance of the healers, but also on the performance of all the other classes, and the difficulty of the encounters. If your raid is going somewhere really hard, is somewhat short on healers, and the tanks and dps are undergeared or otherwise underperforming, all the healers can heal to the maximum of their capacity. In that comparison holy priests score quite nicely on a healing meter, and even discipline priests do well, in spite of the healing meter not counting their shields. But if you are in a farming raid, with more than enough healers, and the tanks and dps just rolling over the enemies, the nature of healing changes. There just isn't enough damage to heal, and healing becomes a competition of who heals the fastest. In that situation shorter casting times and heals over times easily outperform slower, more mana-efficient heals. Thus druids and paladins top the healing meter.
The danger is of course that if a raid leader realizes that he has too much healing, he will be tempted to invite one less healer and one more dps. And if he isn't very well versed in healing mechanics, he might well cut the guy at the bottom of the healing meter. Which explains why healing meters are taken so seriously even by people who know that those meters can't measure real performance. Even a damage meter isn't perfect, but at least there is no limit to how much damage you can deal; healing is always limited to how hurt the raid is, and "avoiding overhealing" is often neither possible nor even desirable. So when Ghostcrawler announces "We have some exciting changes planned for priests", I sure hope that these changes make my priest score higher on the healing meters. Not because my *real* performance can be measured that way, but because other people will judge me by that. Getting kicked out by Mr. Stupid from a pickup raid for underperforming hurts, even if it was just him being wrong.