Community Wiki is:
- A way to mark a question for collaborative editing
- All questions/answers may be edited by users with sufficient rep, but CW makes this purposeful
- Useful when information is constantly being added and updated
Community Wiki is not:
- A way to make otherwise problematic questions (list questions or recommendation questions) acceptable
As above, Community Wiki is useful when information it constantly being added and updated. This is why posts are automatically made CW after 10 edits or after being edited by 5 users. Posts should be manually marked CW when intention is to have this collaborative editing. This is normally done to questions, to ensure all of its answers will be CW.
Community Wiki posts stop giving reputation (due to upvotes) to the original poster once they are made CW. When you have 90% of the content from a post coming from other community members, for example, why would you want rep going to only one of them? CW neatly solves this problem.
This is why CW is bad for opinions, recommendations, and list questions. If someone provides a useful opinion on an aspect of music theory or recommends a method of practice, they should get reputation points if others find it useful! Those do not need to be collaboratively edited, and neither do lists; a new poster will just add a new list item as a separate answer.
Examples of Good Community Wiki Questions
What are "Community Wiki" posts? (Meta Stack Overflow)
This post is a good example because the nature, purpose, and application of Community Wiki has been repeatedly revised. As such, the answers have needed to be revised over time, regardless of whether the original asker and answerers are around to update their old posts. The appropriate knowledge should be added by whoever has it at the time.
How do I root my device? (Android Enthusiasts)
This post is a good example because new Android phones and tablets come out all the time, in addition to software updates. The methods for rooting are not the same between devices and software, and so they are constantly revised. Anyone who reads about a new rooting method or tool can post it in the answer. This also is an incredibly useful resource because it eliminates the need for hundreds of "How do I root Device X with Android version Y?" questions; instead, we can gather it all into one cohesive wiki.
Below is the previous version of this post.
Community Wiki was revised relatively recently. In general, CW-izing questions is discouraged. For the most part it has been abused, acting as a "stamp of approval" for bad subjective questions.
Only moderators (and 20k+ users?) can make a question CW. This is reserved for the rare "list question" where answers can still be objectively validated and where the user has a legitimate problem that needs to be resolved. While the example you give meets the latter criterion (there's a problem), I don't believe it meets the first. An example that does meet both is, in my opinon, Well-known composers or piano pieces generally suitable for someone with small hands?.
For more information on the "new CW", see What can we do to make Community Wiki better?