I don't quite get people who really dislike DCL would come to a forum to post about it, either. But, I do understand why people who DO like DCL would come on the forum and post about it. I recently posted a lengthy review that I know had some negatives mixed in with some positives. Part of the reason for posting the negatives is to warn other people away from aspects that they might be disappointed in. Part of it is some hope that these things would be improved on. I don't really think Disney closely monitors these boards, but I've seen many times where the "chatter" in online forums does in fact make it up to the level where companies take notice, and it wouldn't surprise me if some people at Disney aren't at least vaguely aware of strong reactions on the boards.
As for the comparisons, I think a lot of it is like comparisons of Disney parks with others, such as Universal. We love traveling to Disney Parks, but I'm glad that people told me about how good the Harry Potter area of Universal was (in my opinion, that part of those parks beats anything I've seen at WDW, though on the whole we still prefer Disney). Similarly, I think it's useful to know that people who have liked DCL also found things they liked on other lines. And, just as I think Universal puts pressure on Disney's parks to improve, I think other lines improving could push DCL to improve also, which is what I'd most like to see. Also, I think comparisons help in both ways - I can hear about things that let me know that DCL really is the better option for me: I compared my Spring Break Fantasy cruise last week with a colleague who cruised a different line/route, and the differences were significant; I would not have wanted to be on that cruise, despite the significant $ savings. We go on cruises infrequently enough (once every 2-3 years) that just "trying out" other lines is not something we can afford - I appreciate the ability to compare something I know and like (DCL) with something I don't know much about. Based on what others have told me, for instance, I now know one line I won't plan to cruise, and one I ought to consider if we ever do something besides DCL. Without some negative points posted, I wouldn't have even known that.
As for the comparisons, I think a lot of it is like comparisons of Disney parks with others, such as Universal. We love traveling to Disney Parks, but I'm glad that people told me about how good the Harry Potter area of Universal was (in my opinion, that part of those parks beats anything I've seen at WDW, though on the whole we still prefer Disney). Similarly, I think it's useful to know that people who have liked DCL also found things they liked on other lines. And, just as I think Universal puts pressure on Disney's parks to improve, I think other lines improving could push DCL to improve also, which is what I'd most like to see. Also, I think comparisons help in both ways - I can hear about things that let me know that DCL really is the better option for me: I compared my Spring Break Fantasy cruise last week with a colleague who cruised a different line/route, and the differences were significant; I would not have wanted to be on that cruise, despite the significant $ savings. We go on cruises infrequently enough (once every 2-3 years) that just "trying out" other lines is not something we can afford - I appreciate the ability to compare something I know and like (DCL) with something I don't know much about. Based on what others have told me, for instance, I now know one line I won't plan to cruise, and one I ought to consider if we ever do something besides DCL. Without some negative points posted, I wouldn't have even known that.