I usually get at least 1 com as Tank or Healer, 1+ com in single run is quite often for these two role too
as a DPS, getting a com is about 60-70%
I might have some explanation for it, as a Tank, I have to focus on building aggro to each mob in large pull, keeping my CD pop at right time, and dodging AOE, so as a Tank, the healer performance is more noticeable to me, the same way goes for my Healer, I have to prioritize healing tank, so my focus will be on tank
but when I play DPS, all i need is focus doing my rotation, popping buff at right time, using utility when needed, I barely notice other DPS, unless the run is getting very slow, not to mention, utility from DPS isnt very noticeable when everyone focusing on their job