Demos are files that contain information about what happens in the server, so when you play a demo you are basicly creating a server and doing everything the file tells it to do.
What Viciouz app did was that it opened dp, called the dumpframes(?) command, and run the demo. The dumpframes command makes dp save an image of what its viewing every frame. So then Viciouz app would just gather the images and create an avi file.
I think making the built in feature would be nice, as long as it doesnt remove the old demo support.