I think the point of the Unity victory is to rush research and learn it before any other mages have the ability to counter it. I've come very close to losing to a game because an AI started casting unity before I could counterspell it, but I got lucky and learned it a few turns later. Though AIs also wont counter it if you are at peace, wont apply to multiplayer but I guess it's a nice way to end a game where you don't feel like killing everyone, pay for a peace with the remaining AIs and cast the spell at your leisure.