I like the concept, rather different than the migration to 32bits boards going on now elsewhere.
Apart from lowering development costs separating motion control from everything else just sounds like a good idea.
It would seem to be a nice evolution for Marlin/Repetier and current ATmega hardware too: motion control only and loose everything else. Add a Pi for the rest, preferably including your JSON interface because that is just everything one could ask for in an interface :-)
All in all great job on the UM3 development guys!