I'll try to be a bit more specific! I use The Bat because:
I have tried every IMAP client I can find, and I cannot find another client that will reliably update the folder count for virtual folders.
I do most of my communicating by email (I'm old-fashioned that way) and I live in a virtual folder ("saved search" in Thunderbird-speak) which I call "ToDo". That folder contains all of my unread mail and all of my starred (flagged) mail.
I can create a similar virtual folder in Thunderbird, but the folder count does not update reliably. I often have to refresh the virtual folder manually. Outlook won't even update the folder count reliably on standard IMAP folders.
This alone is enough to tie me to The Bat. UI-wise, I'd far prefer to be using Outlook, or even Windows Mail.
EDIT: I must admit that the last time I tried Outlook was the 2007 version. I got bored paying for upgrades, and I figured if they hadn't solved something as simple as updating IMAP folder counts by then, they were never going to do so. I am happy to be told I am wrong.