Since the typing is made with the full privileges of the current user and he might be totally unaware of this, it is nearly as good -- the dongle has only to wait until some administrator is on the computer. It gives all sorts of new attack vectors at least.
That's true, and people should be very wary of plugging in USBs. I, for one, would like the option to sandbox usbs: when it gets plugged in, ask my goddamn permission before granting it driver privileges. I'd probably be ok with a keyboard one, but write-only. But that doesn't mean USB is an inherent security risk, like something with DMA might be. One huge benefit of the serial bus is that by serializing the operations, you don't have to grant it access to bypass the cpu.
