swfdec handles way more than just youtube & co videos. For instance, most commercial ads work just fine (we really needed that :)). On the other hand, on the video side, RTMP protocol isn't handled yet, which is what some flash players stream from. And there are also some video players that won't work, like wattv, dailymotion and a couple others.
And like it was said, swfdec is suboptimally designed for speed, which is a bit problematic when video is involved.
So, i repeat. There's nothing really revolutionary or great with this flash plugin. It's mostly a test of the plugin system for me, and an evaluation of swfdec speed and compatibility for you when it's released. :)
But it's still cool to be able to say you can watch embedded youtube flash stuff (even though getvideo+mplayer is way more efficient :)).
I think RTMP is not very easy to implement. But I think I heard something about it getting support in mplayer, so hopefully that will solve it sooner or later (I don't think RTMP support is crucial at the moment, but it's probably getting more important).