Well, if you're already on BT Broadband and have a 360 then it's a no-brainer. All of a sudden you've gained a library of on-demand movies and Premiership games. Nobodies going to subscribe to BT Broadband just for that - it's just a cherry.
After being their usual bombastic selves about IPTV, MS are realising how different the demands of the market are with respect to reliability and responsiveness. These are challenges the Xbox team have already faced (and failed, and fixed!
). The rollout of BT Vision is the first major implementation of their system. This Xbox 'option' is another baby step along the way.