It means you are prohibited from using your own code except as how the FSF sees fit. That's what copyright assignment means.
So if you want to take your code and contribute it to a BSD licensed project? Nope. FSF won't allow that. Or use it in a commercial product? Nope. FSF won't allow that.
You can use it in a GPL product, so long as you keep the FSF copyright.
> A good weapon can be effective as self-defense merely as a deterrent.
Sure. And what about Samba?
Nothing prevents the Samba team from joining together collectively and suing someone for violating their license. The fact that other people have contributed small pieces to the code doesn't matter. A GPL license violation is still a violation of the code they own.
e.g my own project: FreeRADIUS. At this point, I've written at least half of the code. If there's a licence violation, I don't need anyone else's permission (or copyright assignment) to go after the violators.
So what, exactly benefit do I gain by submitting code to the FSF, and assigning copyright?