it's really quite simple. the comments exist for the benefit of the skin author, as a place for feedback on their work. the ratings exist for the benefit of the downloaders.
as an artist, this has a huge benefit. when getting feedback about your work, in hopes of improving what you do, or to find out what it is you're doing right, quantitative numbers are the worst information. forcing a person to tie a rating and a comment together would do several things, in my opinion:
-reduce the amount of thought going into the comments
-force the comment to be qualitative rather than informative
-discourage people from making comments about various details of the piece which might not have anything to do with its overall quality
my advice to artists is to -definitely- not pay attention to the rating, but rather to read the comments.
additionally, if it's merely a desire to find a way to increase the number of comments, please remember that quantity is far less important than quality when it comes to critiques.