I've noticed, that a majority of people who really need improvement on their DPS numbers don't like to be told they are under-performing. Many will hide behind the fact that the content was still cleared, or they had such and such to deal with, but at the end of the day, its someone telling them that they can be better and really excel at their job and they're throwing the baby out with the bathwater and just getting offended at any kind of criticism it would seem.

I firmly believe, if there was a public in-game parser everyone could see, people would grow up quite fast and establish much more realistic and pragmatic views regarding their potential as DPS. Developing a thick skin is a life lesson.