I'm not sure I understand why extra reputation is required to be able to comment on questions and answers. (At my level of reputation--16 in CrossValidated, though higher in some other areas of StackExchange--I can't do this, although I can ask and answer questions.
Suppose that I want to add to a question, or ask for clarification of it, or ask for clarification of an answer, when I don't have an answer myself. I can't answer the question, and it would be counterproductive to start another question thread. It seems as if commenting ought to require no more reputation than answering.