Mach 3 has a clever trick, they hack deep in to Windows and take over a hardware timer so they can step in real time. Windows hates this and got better at protecting itself with each new revision which is why MACH doesn't work passed XP.

If you go USB then you lose all the critical timing and you have to send a pre-digested sludge down to the machine which includes the step rates and makes MACH somewhat reduntant.

Hassle is, MACH is the hobby industry standard and as soon as you start pre-digesting you step out of the comfort zone and chain yourself to a single supplier who may go down the tubes leaving you high and dry. MACH is unlikely to demise until a new standard is accepted. It's catch 22.

Having said that, I drive my mills via USB on Win7 without any restriction on running other programs at the same time. Do you have the pioneering spirit? You may end up with arrows in your back