I'd imagine the convenience and portability of having just one device. Even then I can supplement this machine with an iPad Mini for pure consumptive purposes (i.e. this can be an amazing convertible without having to be a be all, end all device).
That doesn't make sense for those of us who actually use their Macs to get work done. I think Apple is smart in keeping tablet functionality away from the Macs. The rMB in its current form is perfect.