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?
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.