Here at Electric Spec, we do use a "form" rejection letter, which we modify in certain circumstances. From time to time, I also include specific comments about the submission. If an author wants to share my comments with the world, fine with me.
The issue of additional comments an author might make about the rejection gets a bit trickier. If an author has a legitimate commentary, fine. I'm a big boy--I can take it. On the other hand, if an author uses it to take unwarranted cheap shots . . . well, lets just say I probably won't be very interested in anything else the author submits in the future.