As unfortunate as this is, it's actually just making something that is already possible easier. The extension will only you to download a video that is already going on to your system (cacheing or a video that you could do a "save as" to anyway). It doesn't seem to affect a video that is streaming.

I tried to see if I could get a .wmv streaming file or a .flv streaming file and it would not pick up either. However if I go to a page with direct urls to videos that cache or progressively download as you play them then yes this extension will work.