We're getting a bit offtopic here but hey, at least we're talking about animation frames.
Definitely automated tweening has its disadvantages, I'm more than aware of that. Point being I haven't had any real tests yet. If I have a one second animation consisting of 15 keys I don't think it would look that bad if there was another 15 tweens making it look smooth even though half of the frames wouldn't have the same kind of quality. I'm not talking about taking that kind of animating shortcuts as I tend to have a lot of handdrawn keys.
Taking this project's overall looks into consideration it could look very cool to have morphed tweens there too since there is no way I'm animating 60 frames per second by hand. I'm okay between 12-18 keys per second. Professional handdrawn animation in cartoons and anime generally tends to have 12 animation frames per second (which is naturally every other frame of a broadcast fps video file), maybe the full 24 frames in fast scenes.
<img src="http://img7.imageshack.us/img7/8183/tracedkeys1.png">
Here's an example of the said keys. It's a simple fast animation but the problem lies in making it look smooth in a very detailed environment that has a lot of prerendered materials. If it's played fast enough to make it look smooth it's simply too fast and doesn't look natural. If it's played too slowly it stands out. Morphed keys could easily fix this by providing more sense of motion in the animation between the keys.
I put an overlapped "onion skin" version of the animation and I don't think that it would be reasonable to have full drawn production quality animation frames more than that on a character sprite.
Edit: Killed a typo
Edit 2: Oh and as a reply to madster's hint on trying Premiere... We don't exactly boast a budget that would allow us to buy a 1000� piece of software for animation tweens. Thanks for the heads up though :D