Well some of the new Microsoft Lumia phones had the ability to work as a desktop computer. Look how well it did for Microsoft ;p

The idea is cool but the computers and smart phones are so cheap everyone has somekind of computer (or access to one) + smart phone. This would've awesome in the mid-2000s, though ;-)

+ Haven't seen a device that really solves different usage patters of mobile phone and desktop in the same UI. If you're going suggest Windows 8-10, I'm sorry to break it to you but they failed miserably.