I can't speak for everyone, but I've experimented with Thunderbird because: (1) I didn't want to trust a third party with all of my emails, (2) I wanted a clien

I can't speak for everyone, but I've experimented with Thunderbird because: (1) I didn't want to trust a third party with all of my emails, (2) I wanted a client that supported PGP, and (3) I wanted a client that would work offline when traveling. I've since given up on PGP and offline access for the most part and have been using FastMail's web interface.