I like the concept and considering there is no fee, it's worth the effort to register a bike. You certainly have nothing to loose.
It isn't clear to me how lost or stolen bikes are spotted by others though. I see hundreds of bikes in my travels. Without checking & recording serial numbers, I don't see how anyone would know a particular bike is stolen. The registry does give potential buyers of used bikes a place to go check serial numbers but that isn't always possible when buying online from sites like eBay.
I guess the system must work since they claim to have recovered over 5000 bikes.