I was a big fan of The Bat! for years, and I still think it's one of the best clients out there for "old-school" email use. I think I just got to the point, where for my personal use, there was no real change between versions, and I was still buying upgrades every year. I know I could just stick with the old version, but I always used the upgrade discounts. I tried Thunderbird at some point, and it worked fine for what I needed, plus had way better search at the time, so I figured I would stick with that and save the money.
My only real complaint about The Bat! was the proprietary format the mailboxes are stored in. You can't use things like MailStore for backups, and if something corrupts the mailbox you're pretty much screwed if you don't have a recent backup. As I remember it, it was not straightforward getting your email out of The Bat! and into another program either.