[Dimaz]

Actually in my experience AC never was a game winning spell. It hurts the caster almost as much as everyone else, and there are some nations that are very resistant to horror attacks (returning, mandragora bodyguards, ordinary bodyguards + frozen heart, etc) and with proper measures (that require tons of micro however) they can outplay the caster of AC.
Of course if you are already winning it can help you to do it faster because anti-AC measures can be quite expensive. But if your nation is of the same power level as others, I wouldn't recommend casting AC if you play with experienced players.