I can't speak for others only myself. I have a BT line and broadband with sky as Virgin Media don't have cable in my area!
I use voip as I need a second line at home; BT wanted to charge me a whopping £130 installation, an additional £16.99 a month line rental and lock me into another 12 month contract for a second line. Voip was a no-brainer really. :)
You don't really make much of a saving taking only Virgin Media'd broadband package, I think it costs just £3 a month more to take a PSTN line with their broadband package. Trouble is here in the UK you have to pay extra for things like caller display, call waiting etc. (That goes for all PSTN providers)
Virgin Media are also expensive on their call rates.
So you can save money using voip there.
Actually a lot of Virgin Media's customers do make use of voip even though they have a Virgin Media landline.
I guess here in the UK the real benefit of voip is business use, explains why hardly any voip providers here in the UK push domestic voip services.
Mobile data and voip comes in handy if your out and about and want to take calls without having to pay for the cost of a diverted call to your mobile.