Except that it really isn't. Wyld lays out a few reasons why, the most obvious being that one player outputting tons of damage doesn't mean your group actually succeeds. As a resto Shaman, I often get the short end of the stick on damage meters-- I do both healing and damage, so I never end up at the top of either list. Also, Earth Shield still isn't listed correctly even in the latest version of DamageMeters-- all of that healing, which I'm clearly responsible for, gets listed as the warrior's.
That's more of a bug fix, though (DM just has to get its numbers straight), and Wyld has bigger ideas in mind for damage meters.
First, he wants to build an actual tracker right into the official client. I can't say I'm in agreement with him on that one-- having written about WoW for a while now, I know that most people (probably the people who aren't at the top of the list for whatever reason, but especially raidleaders) hate damage meters with a passion, for exactly the reasons stated above. But we'll let this point slide, because, as I'll say in just a second, Blizzard won't ever implement it anyway.
More importantly, Wyld wants damage meters to not just assign a number, but a uniform rating. The question shouldn't be how much damage you're pumping out, it should be how well you're doing your job, and Wyld would have damage, crowd control, healing, and other actions all assimilated into a "group contribution score" that you could then use to compare to others of your class.
It's quite an idea, but something like that seems like you'd need a genius to code it-- as often as the skills change, someone would have to keep up not only what every single action is worth, but also figure out a way for players to compare their own scores to the average. I have no doubt that Blizzard could do it (they are geniuses), but I don't think they want to-- squishing gameplay into numerical terms is the opposite of the experience they want to create. DamageMeters is helpful for some players who push themselves to be as good as they can be. But for people who aren't interested in the math, fighting for a "group contribution score" (rather than, you know, loot or XP) would lose some of the magic.