What benefit is the practice of voting down? Why should genuine comments about a lesson be voted down at all?
It strikes me as a very 'negative' practice and not at all helpful!
It is most discouraging when people are trying to be constructive in their comments!
If you're asking about downvoting the sentence discussions themselves, then the answer is a clear no benefit at all. Especially since they become unsearchable once they are below zero. Upvoting good comments to make them show first and downvoting incorrect answers however is ok.
Please upvote any sentence discussion you come across that has a minus vote.
Many people are used to negative thinking.
That combined with the crappy buggy downvoting system in the duolingo forums is very bad! Very many positive constructive postings are made invisible within minutes because there are five downvotes, and those downvotes could even be from one single teenager who is bored and who uses multiple accounts.
On a positive note, the learning modules of duolingo, stories, gamification, etc. etc. are excellent!