The copyright assignment papers you write aren't as unilateral as you might think. It doesn't say "this belongs to the FSF now, none of it is for you." They're fairly clear in the language and say that the FSF will defend your software and keep it free for everyone, including of course yourself. It's an exchange: you give them your copyright, in return they will use your copyright to forever defend your software from becoming non-free. So your software will always be available to everyone, including yourself. You also only have to sign them once ever; it's not like they require new papers for each contribution you make.
Oh, and you also get a sticker, I think. :-) Due to some legal finnagling, since you're signing your copyright away as part of a contract, you need to receive something of tangible value in return. I think that's why you get back a sticker (or maybe it was dollar bill, I forget).
I don't think the FSF has ever gone to court defending copyleft, but that's not entirely the point. A good weapon can be effective as self-defense merely as a deterrent. It doesn't necessarily have to be fired. Also, the FSF always exhausts all other means to bring about GPL compliance and considers legal action a means of absolutely last resort.