With the PC (or Laptop i suppose) the internal components can still be easily swapped out, to keep up with new technologies, or to add enhancements to the overall system.
A Tablet I take it you could fit directly to the dashboard, in the Stereo recess, or other location - but the external connections then, for example Audio, how would that work? You'd just connect directly to the 3.5mm Headphone jack of the Tablet? Would this be decent enough to cover the Audio install though? Surround sound? BASE etc? You have no options for interfacing other Video devices, ie: a Camera?
Can see the point here though, with the Tablet, you have WIFI / GPS / BLUETOOTH, even the Operating System, all already built into the one tidy device - no additional wires / components to mount, or factor in. Infact you dont even require the seperate, dedicated Car-PC screen!
Thinking about this, the Tablet MIGHT now make more sense.
Granted, the 'lazy mans Car-PC' - but it offers alot of advantages.