While the idea to copy a close reason to a comment is not new, I did not find a suggestion to implement it as an optional feature. Let's have a check-box (or something) that can indicate that a person voting to close would like to have a copy of the selected close reason to be posted as his/her comment. Such option would be disabled by default.
I think it will add a convenience to those that chose to use it and will cause no problems to all others.
