As has been discussed some time ago, a lot many answers are in the comments. If the users who have commented on the question do not write the answer to that question, can someone else use the ideas in the comments to write an answer and post it as community wiki ? Should we encourage this practice especially for older answers ?
Yes it is. It is polite to cite the original comment and you should also expand it a bit so it becomes a proper answer but answers should not be given as comments. See this (and various similar) questions on meta.so for the "official" policy.
Basically, first leave a comment asking the user who left the comment to post an answer instead. If they don't, you can shamelessly steal their comment and post your own answer.
Remember that the objective of all SE sites is to build a repository of useful knowledge. Answers should be answers so when you do this you are helping the site.