The overwhelming majority of laptop users only use their computers in places that have wifi, so a mobile data connection isn't something they'd be willing to pay for. Plus, for those who do want to use their computer in more remote places, tethering to a phone is a perfectly acceptable solution.<p>In other words, too few people want one to make it worthwhile making them.
Whilst tethering is an option it would be more convenient for the laptop to have its own HSDPA connection. My phone uses up more battery power when I have the hotspot function activated, so I generally don't have it turned on. As for WiFi, when you are on the move, having to connect to different WiFi networks becomes a chore.<p>When using my iPad I really do like that fact that it automatically connects to the 4G network without me having to do anything. So I would buy a laptop with a SIM as long as it runs OS/X or Linux.