Hi David,
I work on the team responsible for developing the virtual firmware, specifically the virtual EFI implementation.
We had not intended that EFI be the default. We realized that we'd made a mistake too late to correct it in time for vSphere 5.1 GA, and the consequences of the initial mistake had propagated to various other places which had now assumed that EFI was intended to be the default, such as documentation and release collateral.
The primary reason for wanting to return to BIOS by default is the lack of FT support – We did not wish to provide a default configuration that was going to be incompatible with FT. Secondary reasons exist, such as a small number of PCI Passthrough scenarios which would work on BIOS but fail on EFI, and generally broader support for BIOS in the ecosystem – such as guest OS deployment solutions, OS recovery solutions, PXE boot environments and PXE server support, and so forth.
That's all there is to it. It was a mistake which propagated in a way that we couldn't clean up in time for vSphere 5.1 GA, and it's most regrettable that caused the confusion that it did.
My advice: If you don't need FT, won't be using PCI Passthrough (or if you can validate that your PCI Passthrough configuration works with virtual EFI), and have few or no dependencies on other BIOS-specific tools to deploy or manage your OS, you can feel free to deploy EFI Windows 2012 VMs.
Hope this helps!
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Darius