I get many comms as tank, healer, or DPS, but... Yeah, I feel like it's pretty random. Sometimes I'm doing an excellent job as a WAR and I get no comms, sometimes I just get sleepy and brainlag through most of the dungeon, and I get 3. Same as healer, and, for twelves' sakes, same as DPS. And it's frustrating to get more comms for being a sleepy BRD that just uses 4 skills (Note that I don't say BRD is a simple class, it's just that I am more "lazy" with it, since it's... probably my tenth favorite class, so yeah) than for being a NIN that does his very best.

I pretty much feel like comms are given at random. And that's pretty much how I give comms myself. If nobody really stands out by doing something great, or playing its job to the fullest, or simply being nice and chatty, I give comms to... the best glamour, or the job i'm leveling these days, or whatever. It's almost never tanks, though, since as a Main Tank myself, I'm very critic of them.