Downvoting is a difficult Area. I understood you should only downvote plagiarism or abuse etc, not poor quality posts. There is plenty of stuff that would be down votes if you downvote poor quality but what is the incentive? There are no monetary rewards for downvote and as many of these types of posts have a 'fan club' you are opening yourself up to abuse. I've seen individuals reduced to negative reputation because they have posted negative comments in particular comment threads. It's just not worth it. If you expect the downvote feature to work it needs to be looked at.