Rationale :
I'm not "chasing" anything. Windows 7 and 8 are virtually identical when it comes to games, so since they cost the same picking the one that's certain to support DX12 for a new system is a viable option. Though the still-crap interface of Windows 8 will prevent me from actually 'upgrading'.
I was just speaking more generally, I've come across a number of people who seem to think having the very latest DX hardware is the be all and end all to be able to play games when in actual fact most games will still be pervious gen DX compatible otherwise there would be a huge lack of sales.
For example BF4 is only DX10.1 - so DX11 is still here for some time to come.