I share your concerns about the mixing of the personal email and messaging of µBlogging especially since it revolvs around people knowing your email address.
For this reason I doubt it would replace my use of identi.ca. However I think your viewing it as another social silo seems off the mark from what I’ve read so far.
“Public Buzz updates are available as Atom feeds, enhanced with Activity Stream data, published in real-time with PubSubHubbub notifications.”
This doesn’t sound like they are locking in data.
Connecting the service to 3rd party sites seems to rely on XFN, FOAF both open standards.
This doesn’t sound like they are limiting the data entering the service either.
They claim a future API working with the Atom Publishing Protocol and various extensions of it along with the potentially brilliant salmon commenting protocol.
I’m not suggesting Buzz will take off, it certainly has some issues, not least some that you have raised but I can’t fathom the silo criticism. It seems anything but.