True, thunderbird is cross platform and works in general, however Evolution does tend to work better in most cases. I used to use Thunderbird all the time, until I needed to start integrate things with the office. I can accept meeting requests, which goes directly into my Google calender which then syncs to my phone. Now most phones can connect to exchange directly but integrating it all in Thunderbird is a bit more work. Also Evolution at least is working on a Mapi library.
Oh Thunderbird at a time crashed regularly on me, until I moved over to Evolution, at which point those problems went away.
Now I am not saying Evolution is the best, I have a few things I don't like about it, mostly as to how it handle picture attachments. My personal preference for now will stay with Evolution at least.
True, in an office environmnet I guess evolution would be much better. I don't really have a need for that, I purely use Thunderbird/Evolution for email only.
I'm inclined to give Evolution an extended run though.