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The issue here is are you hiring a guide or a babysitter ?
IMO, with a paid DM, your hiring both. Depends entirely on the dive, the divers and the conditions, on how much of one or the other come into play.
Sounds like we won't agree, but again, from my experience, to say that its okay to send a low on air diver to the surface solo just because they are certified and therefore are qualified, is a potential mistake.
I know that from experience. Most qualified divers i have escorted to the surface would have been fine by themselves. Some needed me or a buddy along to assist in making a safe ascent. It is not always apparent at depth who will have troubles. Thats a major reason for a buddy system in the first place.
I ask the question again, do you have experience as a DM or group leader or are you basing your opinion only on personal accountability being a good thing?
One other thing to think about from a professional standpoint. Like it or not, if your being paid to be a group leader, you
are responsible to some degree for the well being of the entire group. How legally responsible you are varys from country to country. Moral responsibility is more of a constant.
Here in the US, if you knowingly send someone to the surface solo and they get hurt, you lose if they sue. Don't even bother carrying insurance to protect yourself. Because it will be worthless. Most policies have the minimum requirement of enforcing the buddy system. Most insurance providers today won't even blink an eye before they drop you. You would most likely be on your own in defending your actions.
Just something to think about Scubakevdm.