In short, my dream is to have one machine with multiple paravirtualized desktop and server guests, one of which has to be a Windows desktop with powerful graphics.

As Windows can't be paravirtualized in the normal sense of the word, I was quite happy when I heard about VGA passthrough, but then I read on. As I understand it, such a setup would mean that the Graphics will be dedicated to one particular guest, thus you wouldn't be able to switch between guests.

If this is in fact so, would someone please explain to me what the purpose/use of VGA passthrough is? I can think of no real use for it. Yes it's a cool technology, but to me it seems pointless.

It's true that it's possible to passthrough individual VMs to separate GPUs, which is also cool, but in the end I should think that two seperate computers would make life a whole lot simpler. Again it seems rather pointless.