if you're a low volume user and get a good signal, a mobile makes lots of sense. 3 years ago, I took out a monthly contract for £7.50 a month (£7.70 now) and that included a free smartphone as part of the deal with no upfront costs. I get 250mins, 5000 texts and 750mb of internet (3G) a month. Does me fine. Indeed, out shopping yesterday, I was very tempted by the deals out there now. £10 a month gets you a lot if you're not bothered about not getting an Iphone. As for my home phone, one call made in 3 months! Total waste of time.
The big problem is that usually if you just want a broadband only line, it can cost more than having one with a phone line included.