I think it's time for SE to finally make this change, and make all area songs and rolls affect the entire alliance rather than just one party at a time.
I know that SE cannot be oblivious to the popular strategy for tackling more difficult content these days, which is to rotate corsairs and bards through the parties over the course of the entire event. This takes a great deal of coordination, distracts players from the actual content, and I feel fairly safe in saying that nobody enjoys doing it.
When people would be swapping bards and corsairs anyway, what's the harm in simply making those buffs hit the entire alliance? It wouldn't increase the number of effects the alliance has, in fact it could even decrease it by preventing those jobs from giving different buffs to the mage party than they give to the melee parties. Bards could still get around that with Pianissimo, and Corsair should probably get a similar ability if this change is made.
At this point in time, there's really no reason not to do this. I think it will allow everyone to enjoy the game more. Please make this change so that the game goes back to focusing on gameplay and combat tactics rather than party shuffling.