In the current version of your question (v3) you write,

I leave this kind of comment on posts where I think that the OP is making simple and fundamental errors in reasoning. I think most of those posts should be closed and that when a post should be closed, it should not receive answers that might encourage the OP to continue posting without thinking.

At the same time, I think it's helpful to the OP to point out those errors in reasoning --- and that one of the most effective way to point them out is to translate the same reasoning into another context where it's quite clear that the reasoning has gone haywire.

These two impulses are both fine to have, but they are mutually exclusive. If you think that a question should closed unanswered so that future similar questions are discouraged, then close it without answering. If you think that the question deserves a response that will guide the asker towards a solution to their problem, then provide such a response in an answer. Please don't use comments as a mechanism to do both of these things.

There have been numerous discussions across the network about why blurring the line between comments and answers is unproductive and harmful; I will hopefully add some relevant links later today or tomorrow, but other folks who have different search-fu skills than I are welcome to contribute as well.

In the current version of your question (v3) you write,

I leave this kind of comment on posts where I think that the OP is making simple and fundamental errors in reasoning. I think most of those posts should be closed and that when a post should be closed, it should not receive answers that might encourage the OP to continue posting without thinking.

At the same time, I think it's helpful to the OP to point out those errors in reasoning --- and that one of the most effective way to point them out is to translate the same reasoning into another context where it's quite clear that the reasoning has gone haywire.

These two impulses are both fine to have, but they are mutually exclusive. If you think that a question should closed unanswered so that future similar questions are discouraged, then close it without answering. If you think that the question deserves a response that will guide the asker towards a solution to their problem, then provide such a response in an answer. Please don't use comments as a mechanism to do both of these things.

There have been numerous discussions across the network about why blurring the line between comments and answers is unproductive and harmful; I will hopefully add some relevant links later today or tomorrow, but other folks who have different search-fu skills than I are welcome to contribute as well.

When I encounter a question which I think warrants some assistance but shouldn't be answered in its current form, I vote to close it and leave a comment like

Greetings! Your question is interesting but in its current form it's not a good fit for our community. Take the [tour] to get an idea of how we do things here.

In comment-MarkDown, [tour] becomes a link to the right part of the help pages.