When someone first creates an account in stack exchange and ask a question, he is only allowed to comment on his own question because of less votes. I think that he should also be given opportunity to comment on comments made by others. This is because, often the first users of stack exchange questions do not show research effort. So the person trying to solve the questions gives only hints. So if the user do not succeed in arriving at the solution, he is not allowed to express his views. If he is allowed to comment on comment on others, then the discussion can go on. Often the first comments are wrong as in Wedge pure product So as in the above example, even if the user1, after seeing the first comment try to say Jim Conant that he is looking for the reverse correspondence, he cannot say that anywhere, becuase the user is only allowed to comment on his own question. So I think that a new user should be also given the opportunity to comment on other`s comment.
I don't know what you mean about commenting on others' comments. But any user can comment anywhere on their own question page: they can comment below their question or on any answer to their question. Further, they can use the @ functionality. Further, they can comment multiple times (as long as they don't try to comment multiple times in 15 seconds).