As a cook I value comment cards. Even those I think are ridiculous give me an insight into the thought processes of that judge and help me understand their scores.
As a judge I can't remember a recent contest where I have *not* submitted at least one comment card, usually I average two or three.
I don't have a specific set point where I use them, rather I will fill out a card when I feel my scores may require some explanation. For example I might make a comment on a 9-9-7 score to advise the cook that a rib was excellent with the exception that there was a bit of membrane overlooked that brought down the tenderness score.