I've been a bit busy since so I only took a proper look at the keyboard today and Scott's right, one port is for the keyboard cable itself. So I'll need to get one of those splitter cables Gorgonops mentioned earlier (assuming they don't charge me 5x the price just for shipping).

As for the hot-plugging I heard the same thing years ago with PCs too. I think it was with keyboards and mice, but probably related to anything serial as well. There was a risk it could short out something.