The thing is that this is in reference not to common mobile devices but to tons of people that actually do need desktop-like power to be re-located very frequently. e.g. having a very powerful CPU and GPU for 4 hours each day on two different places. Tablets can't do it unless the price / over value is insane and two desktops are twice the cost / value plus they have syncing issues.
It's hard to define a mobile desktop, because the only difference between a desktop and a "laptop" really is that a desktop uses ALOT more power....
Otherwise, they're pretty much the same parts..
So, a mobile desktop could just be a laptop that uses a butt load of power, with a HUGE battery, and better cooling.
Laptops have shrinked technology that makes them worse, however, they are at that first only level of shrinking. Go any deeper and it's practically impossible to justify the cost / value for the same performance. And of course there isn't even practically an alternative when it goes to powerful CPUs and GPUs.
Now what is a deciding factor is that small factor desktops are BIG and HEAVY compared even to a 17'' laptop (well, unless you go to insane 20'' laptops, in that case I'd say better carry a monitor and a miniATX around).
So, one can't just say 'replace a 17'' with a miniATX' because they are practically heavier. And they are insanely harder to setup compared to just 2 USB devices.