This answer is based on my answer here.
I support this for questions, but not for answers.
Almost all of the content that needs a lot of editing is in questions. While we should encourage users to ask questions well, the fact remains that some users don't know how to post code, English isn't their first language, etc. It's not very fair to those users who really try but aren't able to communicate their ideas in the best way that others can understand. And it isn't fair to the answerers either since they won't be able to gain rep on the question after it's gone into CW mode.
The reason why I don't support this for answers is that it would further exacerbate the Fastest Gun In The West problem; there is already a 5-minute free edit grace period for the answerer anyway.
The core idea of a wiki is for it to be community-generated and updated content, where many people edit the post to contribute over a long period of time. In this way all the triggers to make a post CW make sense, except there is no bounding on the time density of the edits.
Improving a question (without the actual content changing) isn't really in the spirit of what a wiki should be, and as I mentioned, going into wiki mode through tons of people being generous enough to help format a post really isn't fair to the asker or potential answerers. The wiki-ness of a post should be evaluated after the content is up to community standards, which happens pretty quickly in the case of a new question.