Yes, this is definitely something to discuss. For the record, I've seen many attacking/cheerleading replies in a lot of fics as of late, not just Dark Messiah. That's part of why a thread on the topic seems necessary to look at the concept as a group.

As many of the older members here know, I used to be downright cruel in my replies about fics. So I definitely understand the urge to point out flaws.

Yes, writers need to know what they're doing wrong. If you seriously want to get better, you have to know what aspects of your composition need improvement. But that doesn't mean that the good parts should be ignored. Even though good writers try to develop their skills and hope to learn from their mistakes, nobody likes to be attacked mercilessly. We all want to get better, but we're also human.

As Martin said, mixing the good parts with the bad is a nice way to give constructive criticism. For one thing, it creates a good comparison for the parts you're critiquing; if you say that one thing is better than another, that gives more information than arguing that everything is poor. And for another, it avoids the tone of angry flaming. Nobody likes listening to someone who consistently does nothing but attack.

Frankly, I do think there's a place for the fully negative review. But that's the sort of thing that, in my view, should be done very sparingly. If you plan to ignore any possible good points in a chapter, you ought to have a really good reason for it. I do this occasionally as a sort of "wake up call" when a writer vastly underperforms his or her usual work. When it's used that way, it jolts the writer into taking a second look and paying closer attention. But when it's done all the time, the flood of complaints degrade into incomprehensible white noise.

I suppose it's all about finding a balance. (Maybe in that respect, reviewing is as much of an art as writing itself.) It doesn't have to be perfect; heck, I'm sure we all tilt one way or another in everything we do. That's just human nature. But I think it's good for everyone if reviews point out both strengths and weaknesses. Writers can obviously use comments about their work to improve, and reviewers can develop their critical eyes in finding such points, thus allowing them to make sharper edits of their own work in the future.

I don't think I need to comment on cheerleading, as Martin covered everything I would have said. But in short, reviewing is all about finding the good and bad parts of literary works, both for the writer's sake and for your own. That's my opinion, anyway.