I'd hoped the proprietary RF mouse standards would die after the introduction of Bluetooth, but I guess we're not so lucky. Why have yet another dongle sticking out of the machine, when Bluetooth is usually built in, and if not, it costs less than lunch to add it?

The only possible advantage that a proprietary RF dongle could have is security, because a flaw in a bluetooth stack has more potential to do nasty stuff to the OS. That's not to say that the proprietary USB dongles aren't exploitable, just that they're a low-profile target and enjoy the obscurity that comes with that.