Make comment-list avaliable for wp_list_comments callback, and
comment-form array avaliable to edit
Add comment-list and comment-form to template_tag
This looks like it would be useful for people who want to modify the comment form.
comment_form() has plenty of filters for theme authors to hook into, including comment_form_defaults which allows for manipulation of all default arguments.
Similar case with wp_list_comments(). We just removed the callback there two months ago (cb52da6) since we can now register support for core's built in HTML5 output. Feel free to pass an array with arguments once you develop your theme with _s, for _s itself this is not necessary though, core's output good as it is.