That's fine if you're happy with BT but not everyone wants to be stuck on a contract for internet. To be honest, I don't even know why there is such a thing as a contract for internet access - what exactly are you paying off?
I was a BT Broadband customer years ago when they offered unlimited ADSL. I wasn't effected but I saw a lot of people stuck when BT changed their terms of service and introduced download limits - all the people on contracts were stuck with nowhere to go. I've been considering BT Infinity because the speeds look so good but I really don't want to end up stuck with BT because they're dodgy, unreliable, overpriced and their customer service is the worst I've ever had to deal with. Seeing them force users to extend their contracts just to take advantage of network improvements is not the sort of thing that makes me want to switch.
I've been with Be* for the last few years. They don't have contracts (you just have to give them a month's notice I think), they offer things like static IP addresses as standard and their customer service is top notch. The only problem is, they don't have any clear plan to migrate to fibre-optic any time soon