In my opinion, A question should not be marked Community Wiki if it is possible to write valid, helpful and knowledgeable answers which contribute to the knowledge base. No single best answer doesn't equate to not worth discussing. Restrict marking a post CW if answers have no correlation to their authors' contributions to the community, e.g. Polls and "Best of's" .
Ask yourself if the effects of CW'ing a particular question are desirable:
Rep is meant to reflect helpfulness, level of knowledge and general contribution to the community. If someone writes a good answer to the question, does that indicate a valid contribution? Is denying rep gain for that contribution a desirable effect?
Even if there is more than one valid answer individual answers may still have value, and so they deserve the rep gain when they're upvoted. Marking such a question CW discourages people from putting effort into their answers.
In a poll thread, one answer is as good as another. The value is in the entire set of answers, not in what individuals chose to vote, mark it CW. In a joke thread (although those seem to be getting rare), mark it CW. There may be good and bad answers, sure, but none of them say anything about the answerer's technical skills or contributions to the site. No matter how funny my joke was, it doesn't mean I'm any better at answering questions or that my account should be given broader moderating powers (as happens when you gain rep).
Is there any real benefit of CW outside of joke/poll style threads? I really can't see a use for it any longer. The other functions are largely obsolete with the creation of Meta.
What do you think?