I think everybody who has ever suffered through flash on Linux systems, know how horribly awful it truly can be. Fortunately things have gotten better, but not by much. I still see the flash-plugin crash randomly across several different systems that I use. There is always the choice of using html5 to play youtube videos now, but it requires quite some processing power. In my case that means that I can not use html5 to play youtube videos on my Netbooks, because it feels like turning the pages in a newspaper.
This is where mplayer comes in, since it can play pretty much every video format out there (including flash videos) and it does it really well.