Keys carry a lot of information with them. The posing, natch. The motion and arcs, the antic, action, and rest. All the real knowledge of animation should be in them. When I was at school, I knew this, but I still thought there was a lot about actions in animation that keys didn't have to worry about. Sometimes I'd just leave huge gaps in my stuff and figure the inbetweens will fix how choppy this looks. -_- We all have to learn sometime, I guess.
Inbetweens carry the keys along and, done right, smooths out the animation and gives it fluidity. They have knowledge, but normally only a fraction of it compared to what keys have to carry. I'd done a lot of animations only a few months ago that didn't have enough information in its keys, and had even less in its inbetweens. Most of those you can still find on my website. Seems I've a way to go yet.
While I also have to work on my timing, a lot of the choppiness on those videos are Youtube's fault. Honest.