Chris - not speaking from experience or knowledge here, but from a standpoint of logic.
There should be nothing at the OS level which would dictate that the systems need to be the same version - it would be recommended (in my opinion) in order to simplify support and reduce the risk of issues caused by different patch levels, etc. Having your platforms on the same OS/patch levels takes quite a few variables out of the mix when it comes to fault-finding an issue, making your life, and that of your support agent, a lot simpler in the long run.
I agree with the recommendation to maintain the same version to ease support issues. But there's a difference between recommended best practices and requirements. The way the MobliePrint document is worded infers its more of a requirement than a recommendation hence the question.
The MobilePrint Worker on each Print Server look for MobilePrint Print Queues with matching print drivers. Win 2k12 and Win 2k12R2 restrict what drivers can be installed with the MobilePrint Port Monitor, e.g. the MobilePrint "Pharos PCL 5 Driver" cannot be installed on Win 2k12R2.
You can install MP Workers on different OS so long as all the MobilePrint Print Queues on each Print Server are configured with the same driver names.