I agree with the points made by Willie Wong. But there are cases when a tangential remark is useful: e.g., "This question has been cross-posted to [another site]". Some questions need clarification: "What concept of integral (Riemann, Lebesgue) do you use?".
At the time of writing, there is a workaround which allows everyone to comment on questions. This is subject to change, as the SE team is considering closing this loophole: see "Trivial answer converted to comment" should not bypass the 50-rep restriction
Copy a link to another Math.SE or meta.Math.SE question. In order to not pollute the Linked sidebar, it is best to use a link that is already present in this sidebar. If there are no linked posts, use a link to a Meta question, such as this one.
Paste it into the answer box. Add a couple of words (not many!), for example "Draft of a comment".
- Submit your "answer". The system will not post it as an answer, but as a comment: look for it under the question.
- Edit the comment to say what you wanted to say. You have 5 minutes for editing. (Of course you can prepare the text in advance and copy-paste it.)
On the main site this works even if you are an unregistered user with 1 reputation point, such as "user153296" above. On the meta site, one needs to be able to post answers (5 rep).
To avoid mishaps, one can try this in a sandbox first.