I have recently seen several examples of highly subjective questions which were made community wiki by a moderator, presumably in order to give them some semblance of propriety. The most current example is Who is your favourite game designer?
Now, to some extent I sympathise with the problem the moderators face. You don't necessarily want to just close these questions without any community support, lest you face the accusation that you are being too heavy-handed or authoritarian. You may expect the community to eventually deal with the question. However, in the interim, the question continues, people get points, and it seems unfair. Or maybe you just have a more liberal outlook on subjective questions, and feel that more site content is basically a positive thing. So, CW, and where's the harm?
In this situation, it may seem reasonable to convert the question to community wiki. No one gets any rep any more, the close votes can continue to accrue, and if the community feels strongly enough, it will eventually be shut. Also, on Stack Overflow, there is a history of users pressuring others to convert subjective questions to CW, normally with the threat of close votes if they do not comply (the so-called 'Wiki police'). This is a norm that many users feel they understand.
There are four problems:
Subjective questions are not inherently evil. But they are much harder to write well. Most of them are poor questions, and should not exist. This is described in detail in the blog post Good Subjective, Bad Subjective. This is official Stack Exchange policy.
Converting a post to CW requires moderator intervention. It therefore sends out a strong signal. It says "I am a moderator on this site, and I think this question is ok, because I haven't stopped it, and in fact I am encouraging it to exist." This is harmful. Most users are not very experienced with the Stack Exchange engine, and they are looking for guidance about community norms. This sends the message "lousy questions are fine, as long as a mod can stop by and fix them up with CW."
Community Wiki was recently banished for all normal users. Why was this done? Why can only moderators set the CW flag? It happened to avoid exactly this situation - weak questions being protected from closure by the nullifying of reputation gains. CW was originally intended only for the situation where you have an answer, and would like to explicitly dilute your ownership, and encourage the community to improve it. It is a signal that says 'this content is jointly owned - please improve it'. The reputation feature is just a side-effect. Preventing reputation increases is not the purpose of community wiki.
It is no longer reasonable (if it ever was) to pressure users into making questions community wiki. This is also official policy. The line is now much sharper; for almost all questions, they either should exist, or they should be closed. There is no CW limbo space. And it's not sufficient that a user request their question be wikified either; the reason for the CW needs to be clear and valid.
Please can we stop hitting poor questions with the community wiki flag without careful thought?