Probably something to do with ads, random joe videos have a high success rate and videos from the youtube front page low.
Also, there is more video on the internet than youtube and vimeo, there is decent software done in flash (such as balsamiq mockups) and sometimes it's the only way to get to some content.
I have flash disabled with click2play and I end up starting it _at least_ once a day.
That's entirely a limitation imposed by JavaScript not supporting keyboard access. We'd rather not rely on Flash, but we have to in this case.
Anyway, I was being facetious, but that does show there are things you can do currently with flash|java|whatever that you can't do with JS.
Obviously I'll be happier when it's possible to do everything only with JS.