quoting Nessari:

More that I don't want youtube boxes flying all over pages, and also a bit that they complicate trying to close down browser instances to lower memory usage (until I hunt down all windows and tabs with a youtube video the flash player on chromium won't die for some reason).

yeah they also sometimes have a way of breaking the use of the up and down buttons for scrolling
EVEN when you click outside the video on some nice safe whitespace ... in the most hilarious version of this, you can sometimes be using the down key to scroll a page and without clicking anything the down key will suddenly stop scrolling the page when the youtube window becomes visible and it'll just start turning down the volume in the unclicked, unplayed youtube clip