It will help prevent an error that happens when the VMBus tries to load in the guest OS. I've just made it part of my build process in general because telling Server 2008 and Vista to detect the HAL at every boot doesn't really make much of a difference in boot time. I'm hoping in Windows 7 / Server v.Next that this is just the default anyway.
This tidbit of information is also useful if you end up taking the MCTS: Virtualization certification test.
You can read more about the issue and workaround
here.
