I like the idea, but i dont think it will scale to new technology very well. Phones and components are getting smaller and thinner all the time whereas this concept will always be the same form factor, something i dont think many people would find desirable over the course of a few years (imagine carting round a bulky smartphone from 2008 now). Also, its pretty much stuck with one screen size. It would also get battered and weathered over time and the whole thing would probably want replacing after 2 years or so of use anyway.
Dont get me wrong, the idea of upgrading the unit like a pc is very cool, but i really dont think the average (non-technical) smartphone buyer will want to go to the effort to do so, they would be more likely to just buy a new handset.
Samsung Tocco Lite (retired) , Blackberry Storm 2 (retired), Zte Blade (sold), Htc Desire (retired), Samsung Galaxy Note (retired), Kindle Fire HD 7` (retired), Samsung Galaxy Note 2 (current), Samsung Galaxy Tab 3 10.1' (current)