Some people want to launch movies from Delicious Library, as an alternative to Front Row.
You can do this after a fashion by putting the file URL into the URL field on the "My Info" pane. You can then click the URL field, which will bring up the context menu, which offers to Open URL. Assuming the URL is properly formed and the file being pointed to has the proper default application set, this should launch you into movie watching bliss.
I'm not going to talk about the fact that this is hardly convenient and that dragging files into Safari to form proper URLs is a pain in the butt. I will grant you those points.
Instead, I'd like to bring up an interesting bug discovered by an incredibly savvy user. See, sometimes even though the URL is correct and everything is configured correctly, the Open URL command will inexplicably fail. It was just one of those mysteries.
It turns out, in order for Open URL to work, the item has to have a valid Amazon number set. I don't know why this should be the case, but it is. Luckily, you can easily dupe Delicious Library with the valid (but obviously meaningless) Amazon number B000000000.