I don't think throwaways have to be such a bad thing. They could work if we just made sure the process and outcome are fun and worthwhile. Problems I've seen with past attempts at this thing mainly center around:
1) Too big a scope for the time - No one wants these things to be a project. It should be possible to turn out a bare-bones version of the requirements in less than an hour.
2) Too much of a competitive angle. People shouldn't feel like there's no worth in them participating if someone else is just gonna make a "better" one. We should give everyone room for creativity so things can each have their own charm instead of just being a sorted-by-quality list.
3) By the same token, everyone who participates with a sincere effort should get a wee review. I know for sure that the reason I never got into the Weekly Animation challenges at Synj's old forums was because people only really paid attention to the submissions of regulars or outstanding animators. Things come off cliquish, and that really narrows the idea's reach.
Anyway, hope these points are considered.