I don't agree. The Cover Art Archive hasn't even been around for a long time, and for a lot of releases it will simply be impossible to find cover art in the first place. The amount we do have is actually quite impressive.
I'd like to encourage everyone to add as much as they can (as long as it's the correct artwork, of course), but artwork is really just another kind of data that will never be complete on MusicBrainz. Which personally isn't a problem, as long as everything I care about is correct and complete. And that is something I can make happen myself.