I have been part of the Stack Exchange communities since 2011, and I see people commenting on questions, answers, and comments over minor spelling and grammar errors, and about knowledge about a given subject.
I feel that this is pointless, as there is no such thing as a 'perfect answer' - in other words something can always be challenged and just about anybody can do it.
The point of a given community is to add what you think might answer the user's question, and try to help the questioner out. The editing system may be fine - as long as the poster can choose to accept or decline an edit. But allowing just about anybody to have the ability to down vote and post pedantic comments makes users look bad and defeats the point of this website.
It is also about user rep, not just comments. Just about anybody can choose to down-vote a comment, question, and answer and every vote counts, especially when you're just starting out.
It should be down to diamond moderators to make sure users use spelling and grammar at an acceptable but not too strict threshold. You don't need to know anything really, in order to be a member of a given community.
So I think that commenting on other users' answers is creating a war-zone between people.
It is not needed at all.