Here's my view:
More than one person can post the same message. This might be done by simply saying "Yea, right on!" about someone else picking the post you think is a POTM or it can be an independent nomination with your own reasons.
Additionally a reason or two can be given for why the post is nominated. That does blur over into agreeing or disagreeing with the post. THAT must not be carried on or done to too great an extent.
All that said, we really shouldn't clutter this up with all the extra stuff. Just getting this list of particularly good posts is what we should strive for.