Interesting concept, especially with the recent changes Google has made to GMail/Apps. The biggest benefit/downside that jumps out at me is the complexity required to run an email system this way - Cassandra clusters, LDAP, external MTAs, and web severs all need to be managed. Obviously every email system requires these components to run, but most full-blown systems handle the relationships between components on their own.

TL;DR - Looks great for large businesses or maybe as a platform for a startup, but not as a hobbyist project.