The reason I switched to gmail from Mail a few years ago was because of performance concerns. Of course now that I have an SSD I have far fewer performance problems in general, but even so my mail program needs to be fast and solid. Offline email access would be nice, and multiple accounts would definitely be nice because I seem to lose about 1 very important email a month due to my forwarding scheme from one gmail account to another, but it's gonna take more than a polished UI to make me switch back to a desktop client.