Tim, I agree with most of your post, but I think you are missing something about the typical discussion along these lines. First, I agree we all make mistakes and we should all help out our fellow divers when we see those mistakes. What you are missing is that sometimes certified divers are so incompetent they are a danger to themselves and their buddies. DMs sometimes hold their hands, doing everything for them, allowing them to dive without increasing their competency level. The "diver" remains dangerous at any depth. That is the situation that some, myself included, would like to see change.
A DM is in charge of the dive. He is responsible for the safety of the dive. That does not lessen the responsibility of individual divers for themselves or their buddies.
DMs do assist with classes and often that is their primary role, but it shouldn't be. That role belongs to assistant instructors (AI) not DMs. DMs are used because AIs are so rare. A DM is in charge of a group. An AI helps instructors with classes.