Mmmm I have to argue that the game includes resources for this problem already.

Bard.

Now bard would give up 20% of their damage to keep that Summoner going strong, and this wouldn't even be an issue until late in the fight; most fights offer good down time or Mana Song opportunities in animations.

The things is, the fights are winnable; may very well be a better Team composition without using the Summoner / Bard combo for this reason, but player's were beating final coil well before gearing up.

Maybe to me End Game raiding is more about "The Team" and less about "The Summoner". Every job seems to have some sort of resource limits.