I give it to whomever has earned it, and it really varies by role, job, and duty. A summoner slinging DoTs around like a pro will always get my comm over a White Mage who is just there existing. I understand the impulse to give healers all the comms, and I probably give a plurality of my comms to healers anyway, but they need the Tank/DPS as much as the Tank/DPS need them, so they still have to show some effort.