Both Evolution and Thunderbird are great applications. The fact that Ubuntu will switch to Thunderbird as a default mail client, should not be considered as a discouragement from using Evolution. It'll be just as available, easily installable and well supported.
But setting up an email account is much easier with Thunderbird and that's important. It's also easier to use in many ways. And I've been an Evo-freak and spent several years living in it, so to speak, but I really love Thunderbird as an email client. It has some features Evo just can't compete with, in my humble opinion.
Another thing is that Thunderbird is also well supported on Windows and since we're now getting Ubuntu One support on Windows, I think it'll be nice to be able to sync contact data and bookmarks with both operating systems (and others in the future). For people who dual boot, for instance, this is magnificent. They'll just install LibreOffice, Firefox and Thunderbird in Windows, along with Ubuntu One, of course and then they can just reboot and still have their files, tools and other data just as available. The same goes for people who take their work home, such as students or office workers. Interoperability is key if Ubuntu is going to really hit the masses, and using Thunderbird as default helps in that regard as well as making Ubuntu easier to use.