Because your comments may dissuade people from reading the article who may otherwise enjoy it.
> Do you really think that I'd be poised to launch the next Earth-shattering blogazine, if only I understood the subtle point of this article?
No.
> Or were you just trying to be clever, without demonstrating any insight of your own?
No.
If you're going to criticize someone for "missing the point" you should at a minimum explain what it is they don't get, that way you demonstrate that it is not you who has missed the point