Every time I have ever pointed out specific things I don't like in answer to "Why won't you date me?" (back when I was available) the guy has used my reply to insist that he will change and beg for another chance.

I've had this kind of thing happened to me and have heard similar stories way too many times. For people who want to ask directly for reasons why they've been rejected please remember than an answer is not license to argue the point. Nor is arguing the matter a good idea. You will not argue your way into a healthy relationship- just take the person's reported feelings and update on that evidence.