Look at the very last demo – that’s the final one, which has complete keyboard interaction. You might find it easier to use on a page of its own: http://jspro.brothercake.com/multidrag/demo4b.html
You Tab to items, press Space to select (or Modifier + Space for multiple selections), then Tab to a target container, then press Enter to drop.
Those are the standard keystrokes, described by the ARIA Authoring Guidelines. (Except for drop – the guidelines say to use Ctrl+M, and this script does also implement that – but I didn’t think that was very intuitive, so I added Enter as well).
As to whether people really use things like this with the keyboard – well in most cases, they don’t, because the interface doesn’t support keyboard navigation! If you build it, they will come 
But it’s not about providing for user preferences – it’s really not for the benefit of power users or people who prefer to use the keyboard (although it does have that benefit) – it’s for people who can ONLY use the keyboard. For that group of users, these keystrokes are the most intuitive and obvious choices, because they’re the keystrokes that are typically used for selecting and actuating things.