When you are looking at a proposed dive, I think there are a couple of questions you need to ask yourself:
1. Do I have enough INFORMATION to do this dive? This one can be difficult, because sometimes you don't know how much information you need. For example, Basic Nitrox will tell you how to figure out whether the mix you have is safe for the depth of the dive you intend to do, but it doesn't tell you if you have enough gas (unless you take it from someone like my husband, who includes that stuff in the class), and it certainly tells you nothing about how to execute decompression, should you incur it. The person who has never encountered the concept of gas management may be rather unpleasantly surprised when his buddy has a freeflow at the far end of a wreck, and the two of them don't have enough gas to get back to the upline, because nobody explained how to figure out what you need to do that. Similarly, the person who has never been in a cave or taken a class may be completely unaware that the bottoms of caves may be deep, fine silt, that will blow the viz if you get into it (had someone make exactly that comment to me the other day).
2. Do you have the diving SKILLS needed for that dive? For your light penetrations, I'm sure you had to use some kind of non-silting propulsion technique, and if you were diving doubles, you need to know how to manage failures. If you're going into deco, you need to be able to solve problems underwater, while maintaining your buoyancy and keeping to your schedule. People don't always even think about the issues they could run into, and whether they have the poise and the competence to handle them.
You can acquire both information and skill through classes, through experience, and through mentoring. The biggest problem with using experience alone is that you don't know what you don't know; holes in your information (and in your skills) may not be apparent to you, because you don't know what you actually need. Classes, when taught to standards, at least ensure you have been exposed to the material that the agency thinks is necessary to do dives you are certified to do. Mentoring can be useful, as well, so long as the mentor himself knows what he's doing.
Requiring certain training to do certain dives is also a way to slow down the diver's progression into more difficult territory. I disagree with the OP about, for example, having an OW class include information on deep diving. The new OW diver is often a little shaky, hesitant, or even anxious. He has no sense of how quickly he will use gas.
He does not need to have the stress of his first diving problem (and we all have them!) occurring at a depth that is adding to his worry, or where his ability to respond is compromised by narcosis. Telling someone they have to wait to do a 130 foot dive until they have gotten through AOW and Deep at least ensures that they will have 8 or more additional dives beyond OW class, before they get into that situation.
However, requiring classes for everything is crazy. I have had divers tell me they can't do a night dive with me, because they haven't taken the class. I also recently had the experience of having someone tell me they wouldn't dive stages with me in a cave, because I didn't have a stage cert. Never mind that I took a class that included stage training, and that I worked into doing it and have been doing it for a couple of years
