I like Thunderbird's UI. It doesn't get in the way, and more importantly, I have used it for a long time, so I know my way around it quite well. A lot of attempts to "improve" UIs end with things like Microsoft's Ribbon interface (which I really dislike).
> does not group messages by conversation
Thunderbird can group messages by conversation. One has to ask for it specifically in the Sorting menu, which admittedly is annoying and should be the default. But it is there.
> [client-side spam filtering] is not needed anymore since GMail and its modern competition.
It kind of is, though. Not everyone is using GMail, and my idea of what does or does not count as Spam might differ significantly from what some email provider thinks.
I'm sure there are prettier mail clients around, and by all means, if you prefer one of those, use it. I used Apple's Mail.app when I using a Mac, because I needed to access an Exchange mailbox at the time, which sucks on Thunderbird. The UI looked shiny, I never got upset about it (UI or functionality), but I happily went back to Thunderbird when I got back to Linux.