Mobile broadband used to be mostly sold as a portable way to access the internet. Before the Blackberry became popular having a pay as you go mobile broadband dongle meant that you could check your emails when you were out of the office or on a train. Business men and women where the core market and you certainly didn't get a free laptop.
Recent research by Virgin Media has shown that 78% of their mobile broadband customers predominantly use their dongles at home. This shows how the mobile broadband market has changed; these days it is often seen as a cheaper and easier alternative to having a phone line. Why pay to have a phone line installed and then pay a monthly fee if you have an inclusive mobile phone contract? Why buy a laptop when you can get one for free with mobile broadband?
There are of course reasons why some people stick with fixed line broadband (lets not get into that debate now). People who rent or don't have much of a disposable income are increasingly choosing the mobile option. The fact that they aren't being as mobile with their laptop doesn't surprise me, particularly with the rise in Smart Phones that have a decent 3g connection.