Does the National Bike Registry work?
The point of the National Bike Registry, and similar schemes, isn't so much to prevent theft as it is to make it (sometimes) possible to recover a bike after it's been stolen.  Police departments do recover a lot of bikes; but, since it's usually impossible to trace ownership, most of them end up in police auctions.  The bike registries help to solve this problem, so that if your bike winds up in the hands of the police, there's at least a fair chance they'll be able to get it back to you.  If the tag is still on the bike, the cops (or a bike shop) won't have to look in five or six different databases.  They'll know which one to check.  But I agree it would be much better to have a single national (or better yet, worldwide) database.

I have a nice bike, and I know that there's no single way to prevent losing it to the creeps who go around stealing bikes.  But the more I do, the better my chances are.