Why would MS be on a 3-6mo release schedule when their big money comes from bigcos who are slow to require vetting of any new OS release and aren't going to inv

Why would MS be on a 3-6mo release schedule when their big money comes from bigcos who are slow to require vetting of any new OS release and aren't going to invest the time/effort required to do that more than once every couple years? (works for company only now rolling-out Windows7 as a beta for a small group of developers)