Actually, your arguement works against you. You keep speaking of protect and shell, or cures and haste. These are not white mage specific spells, just as you stated about it being fair with songs. The "specific" spells (like your songs) like viruna, erase, aoe bar spells, all fall under the criteria of being inside party only. Seems fair that multi-job spells can do it, but not single job spells.
Brb starting a thread about "Why doesn't my alliance get my COR buffs?! Can't they see my floating number card things?!"