While the OP made some valid points there are quite a few players who will ask for songs at inappropriate times because they clearly don't know the layout or pace of the dungeon/fight and MP is sadly not infinite. Even I we were to waste our gil on X-Ethers they have a long cool down and if we blow our MP on smaller groups we won't have it ready for larger pulls.
That being said, every DPS should have Invigorate and every mage should have ethers available because you will not always have a bard available and need to be prepared regardless. I personally always eat decent food even in lower tier dungeons in roulette, if one fails to ensure they are prepared to maximize their efficiency regardless of what job they are on then they are no better than a bard who rarely ever sings songs.
That being said sometimes people just get a bit of tunnel vision and forget or don't notice, it's just like when you see a tank who is not keeping track of the mobs on the screen and they wander off to beat on the healer, sometimes they need a gentle reminder. Like all of these situations where you need to correct someone's error or offer up advise, tact is paramount.
Foe's Requiem does not lower or inhibit DPS other than the one GCD you lose casting it, Mage's Ballad and Paeon however do. I'm not sure why SE felt the need to saddle the weakest DPS with a punishment for helping the party but I guess that is the cost for being able to damage on the run.