I believe what happens a lot is the relationships that exist between the roles of the dungeon. I'm a tank. The healer that keeps me alive has more of my thanks than anyone else, because I know that when I'm pulling huge packs of trash, popping cooldowns on rotation, dodging AOEs, and keeping agro off everyone else, I am still taking a beating, and the healer is kind enough to take care of me. I'm also a healer. A dps that gets hit by AOEs that can be dodged makes my job harder. A tank at least has my appreciation if they don't lose agro, because it keeps me from getting punched in the face. As a tank and as a healer, I generally see at the very least 1 commendation from every 4 man I run, sometimes 2 or 3, but almost 99.9% of the time, at least one, from the tank if I'm a healer, or the healer if I am a tank. That simply comes with the relationship between the healer and tank. It doesn't matter how amazing the dps is, how skilled they are, the dynamic still remains that it's the tank who keeps things off the healer, and it's the healer who keeps the tank alive. So it's no surprise that they commend each other, even if the dps were super amazing.
Even in the case of speed running where you push one another to stressful points, there is still a huge appreciation between a healer and a tank for getting through those huge pulls. Call it unfair, sure, but at least when you understand the relationship between them, you'll at least be able to understand why you didn't get all three commends as a dps. Also, I'm sure many people have heard this, but maybe you shouldn't main the one role that almost 90% of the game community mains. Complaints like "DPS queues take so long" are because of this problem. I think people who just want to carry the big guns are just compensating for something.
