I like the concept, I just feel that putting this into practice isn't as simple as it looks. My biggest fear on one of my oldest machines, my prusa mendel is wobble, oh man. When that thing gets going on infill it shakes. It's triangular with bracing where needed to keep it under control. So with that in mind the only thing I see happening is a extremely slow 3d printer/router. Even the slightly oscillation from a spinning bit would give you highly inaccurate milling. There is a reason for milling machines to cost so much, stability and torque.

I will hold off final judgement when examples come out, but it seems to much like a dream that won't quite work out everybody wants it to, not for 500 dollars at least.