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.