I believe that only questions and answers that have been edited by multiple users and that no longer resemble the original question or answer, respectively, should be made community wiki, given that the upvotes or downvotes are no longer only associated with or attributed to the user that originally posted the answer or question. For example, this answer https://ai.stackexchange.com/a/8688/2444 should be made a community wiki, given that its current version and quality is due to multiple users.
In the article The Future of Community Wiki, it is stated in the section Community Wiki is not a "Quick Fix"
Many sites propose using community wiki to allow content that is on-topic and useful, but can be considered borderline or questionable in other ways. Someone notes that a certain class of question has problems, and proposes using community wiki as a quick fix.
If a question is valuable enough that you believe it belongs on the site, chances are you don’t need it to be community wiki! We welcome all contributions which improve the quality of a site and advertise its greatness to the rest of the world. If you allow a certain class of questions, but only under the stipulation that no one can earn reputation from them, you’ve strongly discouraged these sorts of questions. People aren’t going to put in nearly as much effort to ask them.
Instead, strive for quality. If you're unsure a certain question class belongs on the site, don't tolerate the worst examples — demand that these questions be awesome. Questions shouldn’t be swept under the rug with community wiki; they should get the same respect and treatment as the rest of your Q&A. If those questions are something you are uncomfortable showing to visitors … they probably don’t belong on your site.
Many things which "need" to be community wiki simply don't. Sometimes it’s just a matter of understanding the root of a question: "Software to record video games" can be turned into a great question without needing the crutch of community wiki. Or, you may need to break the original question into smaller parts; a rather well-timed Ask Different Meta post explores this very avenue.
Hence, the question What are all the different kinds of neural networks used for? should probably be closed as too broad. However, given that I edited the post to include "I just need a brief overview (1-2 lines) of their applications.", the scope has been slightly limited. So, at this point, more than one user has contributed to the quality and current version of the question, so maybe it should be made a community wiki (according to my belief above).
In the section Community Wiki is primarily for Answers of the same article
If we haven’t said this enough already, questions rarely, if ever, need community wiki. What about answers? We removed the ability for users to make a question community wiki, but left the ability for users to make an answer wiki.
The intent of community wiki in answers is to help share the burden of solving a question. An incomplete “seed” answer is a stepping stone to a complete solution with help from others; an incomplete question is a hindrance and an obstacle to getting a solution as no one understands the inquiry. It is in answers that the goal of community wiki, for the community, by the community, shows its truest colors.
Yet even in answers, true collaboration is scarce. Most of the time, a single individual can provide a complete answer. There are even times where a question looks like it’ll need a massive effort, but one gallant user steps up to the plate with an impressive and comprehensive answer.
See also What are "Community Wiki" posts?.