I’d wondered about this myself. Apparently, there’s a reasonable explanation, but still, those “no comment” reviews are definitely less helpful than the reviews that have something to say.
If you are a Firefox user and pay attention to user reviews and ratings on the Firefox add-ons website, you may have noticed an increase in anonymous ratings. The ratings are left by anonymous user “random six character string”, and only have a rating but no review itself. This in itself is strange for veteran Firefox users, considering that one of the requirements for leaving reviews was leaving a comment up until now.