I'm going to say that I'm in favor of the "voluntary" comment cards. If we make it mandatory for a card on any score lower than, say, a 7 then we will end up with a three number scoring system - 7, 8 and 9. Some judges don't want to fill them out, think that it's too much trouble, won't change anything, etc.
Personally if I give you a 6 or lower I want to tell you why. A 6 by itself won't kill you but two of them ruins your day. Shouldn't you know why I felt that I needed to say that your food was "average" or less?
And we all know that "average" food isn't going to win any prizes