If I downvote, I always get the "please add a comment explaining why blah blah" text. I think this is a bug because many times I will comment and then downvote. So the text is telling me to do what I just did.
This is the usual behavior i have grown in habit of seeing.
Yeah This is a bug which is caused by the feature of suggesting the user to Add a comment. I think the down vote event is not bound with comments and so it shows.
Flowing are my thoughts on this:
I believe it is always a good thing to show the user to add a comment because it could be that what user commented about could be an answer to someone Else's comment or to ask a question from the OP. So the computer has no idea weather the comment was for down-vote or some usual chatter with the OP. So it dosent seem like it could be implemented perfectly.
If the user is shown that always it will force the user to rethink his action and see it was worth down-vote and he should give a comment explaining the cause for down-vote.
Also if user has already mentioned the reason in comment that notification will force him to check whether his comment for cause is appropriate.
Overall it will improve the quality of comments and Justify the down-votes.