I've found every civic to be useful, but you really have to pay careful attention to how you're playing, and where your civilization is, in terms of development.
I think you can work a mercantilism phase into your game, combine it with the right combination of other civics and wonders, and you can have--as others have mentioned--a powerful great person factory. Or four.
With the Sistine Chapel, it's an automatic +2 culture per city. Wonderful.
Once you create the Statue of Liberty you can drop Mercantilism.