I agree. If I've upvoted one of the existing comments on the post, it's quite likely that I've already read those existing comments and decided that one of them already expresses my reasons for downvoting. At that point, there's no need to nag me to add another redundant comment saying the same thing.
While waiting for this to be fixed, I've added a client-side hack to eliminate these needless nag dialogs into the SOUP user script. It's already in the development branch, and will be part of the next stable release (v1.40). For the curious, here's the code:
// TODO: add localized message variants?
var re = /^Please consider adding a comment if you think this post can be improved\.$/;
var oldShowInfoMsg = StackExchange.helpers.showInfoMessage;
StackExchange.helpers.showInfoMessage = function ( elem, message, options ) {
if ( re.test(message) ) {
var post = $(elem).closest('.question, .answer');
if ( post.has('.comment-up-on').length ) return null;
}
return oldShowInfoMsg.apply( this, arguments );
};