I can imagine there are many, in day-to-day use I find Thunderbird to be one of the worst e-mail clients I've ever used. Slow, unstable, makes simple things hard (try to configure an LDAP address book) and many basic features simply don't work (IMAP search for example).

The author of the original message (jch) wrote elsewhere:

"The mailer is the result of an M.Sc. project, and is described in detail in Jan's thesis.

http://trojita.flaska.net/msc-thesis.pdf

In short, Trojitá is a pure IMAP client, written from the ground up to work well over IMAP with no support for any other kind of mailbox. It synchronizes stuff lazily, and (something that's close to my heart), is extremely aggressive about pipelining. I haven't read the source code in detail yet, so I haven't checked how the low level code is implemented, I'll let you know when I learn more. For now, if you're interested, I warmly recommend reading chapters 1 through 3 of Jan's thesis."

Speed. I sometimes try it with my IMAP server (dovecot, around 15GB of email) and it's blazing fast to browse the folders. It also is quite a light feather (~20 MB of memory).

Claws-email is another very fast and very light email client.

The main advantages seems to be performance and more robust IMAP support.