1. There's no guidance for the OP in the case that the deleter *doesn't* write something helpful. They're just stuck. All they can do is flag or, if they can figure it out, edit... but they don't know what to change.
2. There's no guidance for the delete voter to help them frame a good comment. They may not use it, but we need to be sure they have some guidance. That said, default text often times doesn't capture enough of the story to be of any use.
3. The only limitation is that someone can only do it 17 times per day. Post score isn't taken into account, user rep... nada... only whether …