Commends would be cheapened a bit if you were able to give them to everyone. While I know that they're nothing like a trophy, it's the same as giving every single person who participated in something a trophy. In other words, it loses its meaning if EVERYONE gets them. They don't define you as a player, but it still feels nice to get one or two or 3 (or more for 8 person content). It wouldn't feel as nice if you EXPECTED at least one.

The whole "healer/tank" thing is wrong too. I get PLENTY of commendations as DPS. I got my first 500 as DPS for the golden magitek mount before I started leveling other jobs. Can it feel crummy when you feel like you did a good job and the other DPS might as well have been autoattacking the whole time? Or when after a boss fight, the tank says "great DPS" and you side-eye the NIN who died 3 times while thinking "yeah, I wonder where that came from" and get nothing at the end? Well yeah, but you gotta remember that absolutely nothing is expected from people. Don't let that sour your day and just go on your way.