I stopped using stand-alone mail clients about a year ago.
This week I decided to give Thunderbird another try. I'm keeping it simple this time around.
I'm using Thunderbird for a single e-mail account via IMAP. No Gmail. No shared Google Calendar. No newsgroups (yeah, I said newsgroups, which I had running in Thunderbird my last go-round)
What pushed me back to a mail client was the lack of speed in my webmail client of choice, RoundCube, with my mail provider.
So I'm keeping it simple and enjoying the speed and ease of a traditional desktop mail client.
Thunderbird has seen quite an update in its UI since the last time I used it, and that's enough progress for an app that has seemingly been abandoned by its parent company/foundation Mozilla.
As long as they keep it patched from a security standpoint, I don't need any new features.