My unpopular opinion: Any half decent dps that wants to improve should have their own parser running when running serious content. Statics/fc runs needs to always have a parser running to identify potential weak links/areas of improvements. But I would never use a parser to belittle/point out a stranger's dps in a pug run/df/pf. If it's really bad, I'll just leave and suck up the 30 min debuff.
My unpopular opinion 2: Any half decent dps that wants to improve would actually be happy if someone points out that they are not playing to their full potential/could do better/doing low dps. Its often hard to see one's own mistakes, and having others point it out to you can be a great help.