I suspect that the versioning will have to do with CALs and how most folks license the products:
- Windows Server requires CALs to licenses, and as I understand it, a Windows Server 2012 CAL will include Windows Server 2012 and 2012 R2. No need for further investment on the CALs side. In addition, Windows Servers don't manage data the way that SQL Server does - a file on Windows Server 2012 R2 or Windows Server 2013 is essentially the same (there may be some meta-data differences in the underlying OS, but it's still the same file).
- Same scenario for System Center.
- SQL Server is often licensed per Core (or groups of 2 cores, min 2 groups...). I think the issue Steve brought up (attempting to restore 2008R2 databases to 2008, pathces, etc.) will reduce confusion.
All of these companies who have invested in Windows Server CALs (even my small company of 300-ish employees) would balk at having to repurchase a new set of CALs - that we just purchased 6 months ago... (that's a $10k outlay, and we have to have 1 per employee, since our timekeeping system and employee portal run on Windows...)
Just my 2¢.