I've noticed this behavior a few times recently, probably as a means of either avoiding further rep hits (from asking really bad questions) or to obscure who is asking inane questions. I realize editing of closed posts is explicitly allowed on purpose, but can we freeze out this particular edit?
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Totally against this. One, lack of wikification may be why the post was closed; there are cases where a post might get reopened upon being made wiki.
Two, for questions which are deservedly closed, and will remain closed, converting to wiki might be a "way out" for the poster to avoid taking potentially very harsh rep hits. I think that's very fair to the poster. If the question is closed and sitting at a negative score, the community is already informing them that the question is invalid for some reason; forcing them to take further rep hits is redundant at best.