I think it's an incredibly complicated RFE because tags are global, not folder or even application-specific, and can be renamed. Imagine the cascading effects. Let's say user A has access to user B's tasks and user C's inbox. All three users have a tag "foo." User C renames his "foo" tag, which is applied to both his email and his tasks, to "bar." What is the right thing to do?
For some applications, consider the apparently-just-unveiled
Mailboxes: Sharing vs. Relationships » Zimbra :: Blog