That would require the transfer of files into a web server, which, given enough users, would take up a stuffload of space, ESPECIALLY if you're going to include addons in this.
RTB add-ons would just store the RTB add-on ID. Other add-ons would, at least, be deduplicated, so that identical files would only be stored once, no matter how many people have them.
It would also mean players want to have their stuff secure, so then you would have to pay a extra for a secure connection, and you'd also need some form of encryption scheme.
It would also be easy to inject a malicious file into the server if that was made, again especially if you're going to include addons.
The only thing you'd pay extra for for "secure connections" is having a SSL/TLS certificate signed by a verified CA. And you don't need to do that if all interaction is going through your own application (in that case you can just use a self-signed certificate instead and transmit it with the application). Regarding security on the server for the add-ons you're basically just going to have to trust us on that, although I'm 99% sure that the server side is going to be open-source, meaning that you can just run your own server if you want to. One reason for this is the previous problem.
And (correct me if I'm wrong) I don't think there's any consistent way of verifying the blid a key is used for, and verify that it is a valid and correct key, without actually looking at it, because players (and possibly players parents) would want the addon to not actually look at the key.
It can be verified in the same way that regular servers, and RTBC, verify it; by checking the IP/BL_ID combination with the master server.
Poll