Moderators delete comments, or move them to chat. They do that for many reasons which are not the point of this question. We'll assume they are very good reasons, fine.
People keep writing comments, because if you read a post and you have something to write about it, you want to write it, and you do just that.
The "add comment" button is just there, you click, you write. As simple as that.
Sometimes it's useful, sometimes it's not. Sometimes people answer, sometimes a mod passes by and purges.
Admins state that "comments are not for discussion, they are meant just to improve the question: incorporate them, then delete them".
That's a fairy tale. This is not going to happen. Why? Read again the former paragraph: people want to write a comment, people do that.
Commenting something is a natural impulse. You may delude yourself into believing this will change, but this will not. You will not change natural impulses of people.
You have the impulse to write, there's a "here, you can write" button, people will hit the button and write. Yes, I'm repeating the same concept a few times, but that's because I feel you keep missing the point: you are trying to fight nature, and you will always lose.
Fighting the current never works and it's stupid. Fighting a war you can't win. You should be pragmatic, you should open your eyes, look and accept the reality, and adapt to it.
I'm not saying "don't do anything", I'm just stating that what you are trying to do is unnatural and it's pointless to go on: you'd rather have to find an alternative method of dealing with this problem.
So, which is the problem?
Sometimes an OP leaves a comment when he should have integrated it into his post. Ok, it would be better if he did that. Will deleting comments help towards this end? Uh, no.
So, there are tons of off-topic comments and the good ones don't get noticed? By whom? Most of the times, the OP will read every comment anyway: he'll notice. If he wants to reply or integrate them, he can. Of course he almost never does anyway.
A possible improvement would be to have two comment "tabs".
The default tab would be the "chat" one: you can write below this post anything you like, IT or OT, who cares. The goal is to cater the "I must write" instinct here, keeping the other tab clean.
You can select the other tab, which would be called "improvements/suggestions/clarifications" or something along this line. And add your observations or requests there.
What if a user writes a useful observation in the chat tab? No problem, moderators can move it. Maybe even users can request to move it.
What if a user writes something spammy in the improvement tab? Moderators and users can request to delete it. Still, I think this will rarely happen: there's the other tab just for this reason.
Anyone who considers this to be "too complicated" wouldn't even bother to use any of this site's features properly anyway.
The above is just an idea.
What I'm asserting is that you are most definitely doing it wrong. Just look. It's right there under your nose.
I'm not saying that this "two tabs" is the solution, I'm just saying that's it's a possible solution: you might find better ones if you think about it. And this is what I'm asking you: think about it. Stop and think.
I have to be frank, I think you'll just ignore this as every single other suggestion I've given to this network. I think the SE admins are absolutely completely beyond any reasonable hope of reason. They got almost everything wrong, and have built a strong community religiously dedicated to treat every single rule as a dogma, never stopping to think if this makes sense or not.
So, my hopes of you actually listening – not doing something, but even listening – are next to none.
But I had to do it anyway because, as I said: I have to write.