or even just accessibility -- I often download Youtube videos instead of watching them on the site just so if I have a dip in bandwidth the stupid thing doesn't stutter or stop every few seconds. (really true if I want to watch a 720p or higher stream on YT or other sites... funny though that I don't have that issue with Hulu though )
One of the best ways to deal with it -- especially in terms of making 'stolen' vids be advertising, is to just watermark them.
As others have said, there is NO protecting content you put up on the Internet from copying that's going to last more than a week. Hell, browsers like Opera you just 'view cache' and poof, it's there and ready to save... usually even the streaming ones that are supposed to be 'protected'.
You don't want people copying it, don't put it online. As the old security joke goes, the only secure system is one with zero access.