I have mixed feeling about the idea of in future contests requiring contestants to post wips. I can see how it could easily motivate people to up their game when they see what they're up against, but it could also just as easily discourage some from participating if they see works in progress posted that are clearly above their skill levels, at which point they might just figure "why bother".
Anyway, just a thought.
Yes, that is the reason why we don't require it. Some people just don''t feel comfortable sharing at an early stage, fearing that they can't hold up with the standards posted by others, so we don't force it.
However, we keep encouraging folks to do exactly that, which they may feel a little insecure with in the beginning. In past contests, though, we have been able to demonstrate very clearly how well contest entries have evolved once they have been subject to public peer review, and the subsequent tips for improvement - all the while being a great and very interactive and entertaining learning experience for everyone involved. Not to mention the fact that WIP threads are what keep the contest "alive" over its course.
So we keep encouraging it, but not requiring it.