The point is that on steemit downvotes reduce the payout which a poster will receive.
You are right in not downvoting when people use bad grammar, or even post terrible content, I don't fo it either. Not voting FOR their content is enough punishment for those people :)
Downvoting is a very powerful negative reaction, which can and should be used as a means of punishment:
In case people plagiarize content, in case people blatantly flout the comunnuty rules or in case people pretend to be other people etc,...
Ok... so I should flag that one guy who I'm talking about, then?
I wouldn't flag the post you linked to,... He's just bitching, I guess you are right that an honest discussion is better there :)
I have flagged very few posts, one example was a guy who kept on posting nude picture without using the nsfw tag
(It's okay to post nude pictures, but the nsfw tag is part of the community guidelines)
Also I would always recommend to comment the fact that you flagged, and to tell the person why...
That's my personal take on this issue, perhaps other people have other opinions on this...