on my machine only the Z is prone to inertia, all else locks into place when stopped suddenly, of course not if i am moving at 20 000mm/min
So, your X and Y axis operate independently of the laws of physics? That makes everything a lot easier. :)

Dean is 100% correct. If the power goes out, the machine loses position.

I wonder, can't you just see it- its cheaper and simpler. More reliable and easier to exchange in case of fault. I wonder when you Dean as a maker of CNC machines will accept the fact that the controller responds better to your KISS principle.
How is it more reliable? My Mach3 machine has been running for about 8 years, with zero reliability issues. I turn it on, and make parts, whenever I want. It always works.