It would be nice to have, I agree. I am currently using outlook on the tablet, but it is out of line with the rest of the PCs which use eudora and I really don't like outlook.
Having said that, I used eudora before I reinstalled and just wrote out everything with the tip, but I would like an email client (and news for that matter) that I could write in my own scrawl and send as text or the occasional actual handwriting message.
I doubt there are any plans to add ink to thunderbird as it is hard enough getting the tip to work in firefox.
Hopefully something will come along in the not too distant future as I tend to not use the tablet for email or news at the moment, which restricts its usefulness somewhat. If it doesn't, I am going to write something. Maybe that isn't such a bad idea anyway!