The problem with blanket statements like that is that you can always find an example that disproves the statement. In the end, it's all a matter of balance between personal responsibility, professional responsibility (and liability), business model and reason.
From a personal responsibility point of view it is all about what you find acceptable risk, and what you are willing to do to ensure sufficient risk mitigation. This can be achieved by improving education, equipment and experience when necessary, but also by arranging sufficient support around you (think about getting support divers to shuttle stages for example).
As a dive professional, you inherit some of the clients personal responsibility. It is your job to do your absolute best to ensure the client's safety. This also means you can draw a line in the sand if needed. Unfortunately, there is a conflict of interest between telling clients "no you cannot do that" and the business reality of the dive operator. After all, you don't get too many new or returning clients if all you do is tell them "no" all the time.
The thing connecting all these factors is usually money. Clients want to hold on to it as much as possible, dive operators want to get as much as possible (and often that includes doing as little as possible to get it). Somewhere in the middle is reason. If you can find a reasonable price for everybody, and everybody is able to deliver either payment or service while smiling, you can find the required balance.
I used to work with a woman who booked trydive after trydive. She had several health issues, both physically and mentally. This is a woman that struggled with pretty much every aspect of daily live, but still had diving as a hobby. She was so happy and grateful to do the same little trydive of the beach over and over again. When she did it for the 100th time (that is one hundred pretty much identical try dives!) we had a cake and some drinks afterwards.
She always paid the extra charge for the one on one supervision she required, so the business side was covered (I am sure we have given her some discount here and there, but it wasn't charity either).
However, despite the gratefulness, diving with her was a challenge for me (and many others of the team) as an instructor. Her personality and behavior could change suddenly, she might display panic / sheer joy / random tears / huge smiles within 5 minutes. She would require hand holding from time to time, but when you would hold her hand she had episodes were her hand would really cramp up making it pretty much impossible for her to let go. A lot of the skills were pretty much impossible for her to do. She had really good days and really bad days. Needless to say she wasn't everybody's favorite client (although everybody loved her on land). Over time, her, another instructor and me worked out that a lot of her issues were triggered instead of random. Triggers were often based in a lack of self confidence. For example, her buoyancy was normally pretty good, but it would happen she was distracted by something and hit a sloping bottom.If you left her to it, she would ultimately correct it on her own, but if you would ask her to stabilize, she would doubt her own abilities and that would trigger some sort of response.
She was very aware of her limitations, and was really flabbergasted when we suggested to actually give her some more formal training. We started working with her, specifically targeted to increase her self confidence. In about 2-3 weeks, she was certified as a Scuba Diver (entry level PADI diver where you are not allowed to dive without a professional) by the other instructor. He worked really hard with her to get her there, and she did great. She completely utterly loved it and couldn't believe she actually did it. Now don't get me wrong, Scuba Diver was the limit at that point. OWD was simply not possible, because she couldn't meet certain performance requirements. She was unable to get out of the water on her own, and if the sea was anything but flat, she couldn't dive from the boat at all.
We managed to help a severely limited woman to dive on her own, without holding hands, from the boat if conditions allowed, and really really enjoy it. We did it in a way that worked for everybody; her personal limits were respected, as were the limits of the staff, and the financial side was covered as well. Even though I am not the one who eventually certified her (I would have), I helped in her training and it is still one of the most rewarding teaching experiences I've had.
