The commendation system isn't helpful, but it's also harmless and can brighten someone's day to get one when they weren't expecting it.

I'd say anyone who is clearing the high end content in this game is either already using some third party software to evaluate themselves, or simply does not need it. High end content requires a group, and guaranteed someone in that group is using that software and will gladly let the console friends know what they're seeing.

Would I like an in-game damage meter or some sort of Frontlines-style results screen at the end of an instance? Yes, I love seeing if I did better than last time, but I'm not sure how you would even quantify that in something like a dungeon in a meaningful way. Obviously pure damage numbers is not the same kind of useful metric for that kind of content as it is for just a straight single boss fight, and you can't really count a K/D ratio in PVE content