I completely agree that as users leave there should be a way to back up their data and retire the account. Archive that off server or even locally. Once a user is retired it should not count against the license count since it is no longer in use and is not requiring email resources.
For now, I'd recommend adding a user to Outlook, then move the mail from the IMAP server of Zimbra to local Personal folders. Save that file off to you long term storage of choice. It's not ideal, but it will work till the guys at Zimbra get a solution in place.
I for one would prefer a retire style solution so that users can be kept in the system in a shut off state so that they can be reactivated if someone needs customer info for leagal reasons 2 years later. Or whatever.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now