The centralized keyserver puzzles me a little too but it does have some advantages:

1. The email address adds a layer of indirection on top of your upspin directory and storage server addresses. You can migrate upspin providers without changing your identifier in the global namespace.

2. The email address is instantly recognizable to friends and family. It's more user-friendly than introducing a separate decentralized upspin address (like Jabber IDs). You did mention a hybrid approach which uses the centralized keyserver as a fallback though.

A decentralized keyserver still seems more natural to me. Would be great to hear more on the reasoning from the creators of upspin.