As it stands right now, a failure of a tank, and a failure of a healer is quite obvious, even if there are more than one of each in a party. It usually results in a domino-effect of a party wipe, and who the first domino to topple over is right in plain sight of everyone.
When a tank or a healer fails, they'll be booed and jeered, as one would expect. I wouldn't say I'm the worst tank out there, but I've sure felt the pain when I messed something up!

But when a DPS doesn't carry their weight, it's not always so obvious.
Yeah sure there's times when you see someone trying to shoot Bismarcks back from the island or something like that, and you know they're not contributing anything. But often times, you have four or more dps classes smashing away at a target, and if there's a harsh dps check involved in the fight, you fail. You wipe. But there's nobody to point the fingers at. There's nobody to say to, "Hey you could do better. Would you like some tips?" There's nobody to receive the booing and jeering the tanks and healers have to go through.

Not only is a lack of an ingame damage meter report bad for supporting a players improvement, it's also against the equality of roles. There's a reason tanks and healers are usually in higher demand. Because playing them comes as a double edged sword. People love you, but if you mess up, they'll hate you forever. DPS classes don't get this treatment, and can soar on by with mediocrity and never be noticed, be called out, or improve. If a DPS class were to live with the fear of being treated poorly when they play lazily, they may not see playing as a tank or a healer as such a scary thought anymore. And maybe, just maybe, we'd see a more tanks, more healers, and less of a queue time all around.