I disagree. If I'm making cakes and cookies, and everyone says that they suck, then I feel I'd be silly to ignore their criticisms just because they don't have any ideas on how to make them better. It's not the critic's job to know how to make a good cookie, it's their job to tell me that my cookies taste bland. What would make them not taste bland? Beats me, but that doesn't mean they don't taste bland.
Here's the thing...the "how to make it better" kind of criticism is highly speculative. I could say "this image would be better if you added this and took away that", but for all I know the person might have actually done that and the image was actually WORSE. In any case, when I say "it would be better this way", I'm also simultaneously giving a critique of something THAT DOESN'T EXIST. I'm comparing an existing work to something that I simply imagined in my head. And that seems arrogant as all hell, stating that something that someone ACTUALLY MADE is worse than something that I've just imagined in my mind.
Don't get me wrong, I don't think there's anything wrong with offering suggestions on what to do, but that's all they are: suggestions. I disagree with the implication that people who don't offer suggestions should have their comments dismissed outright. Suggestions can be helpful, but the real meat of criticism is "what does this say, does this image work"? And one can absolutely state that an image isn't working without having the slightest idea what it would take to get that image to work. One can absolutely say that an image's statement seems to be incoherent without actually having the slightest thing to offer in terms of what the image should say or how to say it.
Your points may be valid when you consider "art" being viewed by someone who is not a trained art critic, but I don't think they apply here.
If a person posts a photo here and specifically asks for a critique, it's generally because they want to learn or improve. Comments such as "It's just terrible" or "I don't like it," without any accompanying reasoning or suggestion, are unhelpful and don't contribute to the growth or improvement of the person asking for critique. For that matter, neither do the positive platitudes, but at least those don't break down the person's desire to continue shooting and posting.