There is no attempt in my question to discredit instructors or certification requirements. There is no intention to say that there is no room for professional instruction, and I firmly believe in examinations.
My thought pattern was as to why self study, combined with practice with a competent diver, should not suffice to allow a diver to challenge and OW exam if that's the route they choose.
The argument that an instructor knows how to teach is fine, but irrelavent if the exam process is structured properly. Most universities offer exam challenge options on subject matter more complex than OW and grant credit if passed.
Now the kicker, reading through this thread has swayed me towards mandatory professional instruction, not from many of the arguments presented, but by looking at my own argument for no instruction. I implied that scuba is largely unreglulated. Someone else argued that it is self-regulated. The term "self-regulated" usually means that an organization of professionals is permitted by legislation to regulate themselves in order to ensure that they are competent in this. That legislation sets boundaries for activities and processes that must be performed by the self regulated organization. Does this exist in scuba? If there is no legislation permitting self-regulation, then it is unregulated.
Ok, now if it is unregulated, then we need professional instruction way more than DIY learning because stupid things will happen. Using the drivers licence as an example. By law, a learner must pass a learners exam, be at least 15 years old, obtain a learners permit, and be instructed by (at the least) an experience driver of a minimum age. this is regulated controlled entry into the driving world. In an unregulated world, nothing would prevent a diver from bring his buddy out into the cold ocean on a boat, telling him not to hold his breath, and over they go.
So I'm swayed. I back mandatory professional instruction for the controlled instruction environment. Not for teaching skills. Enforcement though means that no shop will grant a C-card witout instruction and exam (this happens) and no shop will rent scuba gear or fill tanks without because as far as I am aware, it is no against the law for me to be in the ocean scuba diving without a c-card.