On smaller maps it is actually quite useful. culture is also critical late game for allegiance and the ability to spread it faster and to prevent your opponents culture from spreading is a powerful economic edge. I think that the research for 30% faster culture spread should be moved from tier 2 to tier 3 or 4. it is only OP because it is at tier 2 it should be moved to mid game.
I agree with you that culture will not win the game, but I think that because the no other faction posseses a way to counter culture early-game it should be moved to tier 3 or 4.