I think that "requiring" a comment will result in a lot more trolling and worthless comments. We used to require text in certain fields for our survey & most of the time it was filled out with garbage. Removing that requirement removed all the garbage text.
With requiring comments to down vote we would have to manually delete the garbage, trolls & pointless suggestions. They are going to be negative comments & it's hard to trust in constructive criticism on the internet.
I think it's better to have it be socially enforced (ie if you as a good user down-vote, leave a comment) so that people browsing will see that it's a common thing to do but NOT have it be a requirement to actually do anything. Otherwise we moderators will be spending a lot of time cleaning up garbage comments.
