I am one of the downvoters to this post, and I'll add an answer.
I'm against this because I believe the bad far outweighs the good in this case, from long debates to users getting attacked etc. If you want to drop a comment yourself, please do.
Another problem, what if another downvoter already posted a comment? Does every other downvoteddownvoter need to post a reason for the downvote?
Take the following old post on meta.SE, for example, A New Code License: The MIT, this time with Attribution Required.
This post was downvoted 533 times as of this writing. Do you really want to read 533 comments to that post thatwhere most of them will be the same?
I can also recommend checking the following posts https://meta.askubuntu.com/questions/linked/2008?lq=1 and their answers. Almost all of them are about the same subject with good discussions.