Marking example questions as community wiki makes the on-/off-topic decision for close-to-subjective questions easier.
Should subjective example questions allow for a community wiki indication?
Subjective != community wiki.
Questions are voted on-topic or off-topic no matter if they should be CW or not. Subjective questions should be off-topic, CW or not.
Whether a question is on-topic or not isn't determined by whether it's community wiki.
I was initially confused about how things work, however, as "what is the best X?" is clearly on-topic for an "Xoverflow" proposal, even though it would/should be closed and discouraged. I now interpret A51's "is it on-topic?" as "do we want it on this site?" or "is it appropriate?"