When the player is not doing enough dps then they should be informed about that happening. One of the reasons why so many players can't even do the basic rotation is because people suck it up silently and that person can inteionally or not do it in their next group...

If too many people of lower skill get in one group and nobody gets called on it, guess what happens? The whole group gets disbanded even if replacing one or two members could be enough to clear. This itself leads to frustration which in my opinion is far worse than any abuse parser could bring. Harassing with the numbers isn't easy, you need certain balls to do it and not many players have the courage to risk the white knights. On the other hand getting carried requires far less effort and as such more players can enjoy doing it in the shadows... transparency is one of the ways to limit the issue.

So pointing it out even if you clear can actually have a possitive effect in the long run - if the person wants to fix it of course.