You stated my main reason. DOS. No thanks.
To expand a bit. I like to be able to use my cam software on the same computer for last minute fixes and adjustments. I mainly use Sheetcam, but have a couple others too. I don't run CAD on the shop computers. I like using software compatible with what I generally use. Mach 3 and everything else I own runs under Winders. Not say that it is better or worse, just that for me, it makes logical sense for my workflow.