I was very close to jumping ship with Mailmate 2 or 3 times. But the only reason that kept me from doing so is that it does not offer an option for local backups.
Not sure what you meant by local backups, but all messages retrieved by MailMate are stored in a plain directory as RFC 822-based EML file in ~/Library/Application Support/MailMate/Messages (which is relocatable from within the app). It also downloads all emails by default (but not the attachment).
Wait - you mean it is possible to properly archive messages on my mac now? Without having to run a local IMAP server? If I remember correctly that was the only solution at the time. Time to take another look it seems.