[photonius]

Well there is a lot of advantage if they go this route (both for apple and consumers). First it allows them to (a liimited extent) merge ios and mac-os around a common platform (also I would expect to see touch screens on mac if they go this route - they can do touch screens without switching to arm - but it seems more natural fit if they intend to merge ios/mac-os). Second (big one for apple) is higher profit margin on macs. Third arm is winning the op/$ and op/watt fight (esp with gpu) so battery operated device should run a lot longer (of course technology is always a moving target so these factors can change over time).

well, Microsoft's version of windows that merged tablets and desktop worked so well that they abandoned it in a hurry.  
A tablet and a desktop for serious use are two different beast requiring different interfaces. Of course that does not preclude a common core.