That's sort of an informal rule-of-thumb a lot follow. Amusingly, the guidelines do specifically say "Please don't submit comments complaining that a submission is inappropriate for the site."

Really, the simplest thing to do is make sure you contribute to discussion in some small way, and then put a rider on for the less contributive aspects you want to voice. That's what the above is really espousing, and while it's not a rule, it does seem to make the discussion flow smoother (and prevents people from jumping in about how you aren't contributing...)