What even is the purpose of all those non melee/blm jobs?

[Valence]

To start, I will say that there is a subtle, but important distinction to make. Your ability to do your rotation should not be impeded by someone else, however, this does not mean someone else cannot enhance your rotation. Running out of MP because you didn't get manasong is someone else impeding your rotation, however, giving someone else mana to increase a song length, while it isn't needed for that job to do their rotation, it helps to increase their value to the raid. Case in point with old Foe's Requiem, you didn't need extra MP to use it, however, if someone gave you MP to increase the song length, that is enhancing your rotation.

Sure thing, but your ability to execute your rotation is probably also gonna be severely hampered by someone else killing you with a mechanic, not healing you, or losing aggro to you. Should we also remove healing out of the game? Or what about that tank that didn't use enough mitigation and gets nuked by a buster and the healer wasn't good enough to adjust, making me the target of the next one? It really messed with my rotation because well you know. I died because of them fucking up.

Short story is, it's somehow not fine when some gameplay mechanics affect everybody in an important/mandatory way when it's on the DPS roles, but it's perfectly acceptable when it's on the support roles. Go figure.