I am an absoulte novice diver. However, I have 13 years experience as a coach in another sport. One of the decisions we make as we plan the "construction" of a player is the training vs. match experience balance.
It is well understood in our craft that in order to develop a person, you must give them a solid foundation from which to work, you must hone their skills, and then you must put them in challenging situations to allow them to grow. I am speaking now of competitive sports. A recreational player will not approach training the same way a competitive or professional player will. The dedication to the craft is different.
Diving can fall in the same category. Someone who's sole intent is to drift over the reefs in 25ft of water may not want, or in fact may not be able, to hone their skills to the degree someone doing cave diving will. And it's not necessarily talent that is the primary factor. It's the environment. I am strictly an open water diver. I am working to change that. However, my forays into caverns this past weekend, highlighted my weaknesses and showed me where I need to work. I would not have ever seen that swimming around in 20ft of water. The environment became the teacher.
I've listened to many good divers talk about others who "have a lot of dives". One thing they frown upon is those who have a lot of the "same dives". If you have 400 dives, and all of them are shallow openwater reef dives, you are not going to be the same as the diver with 400 dives who's done 100 reef dives, 100 caverns, 100 dives below 150ft, and 100 cave penetrations. Time in the water may be exactly the same, but the environment demands more and develops the diver.
While diving is not a competitive sport, it does bear resemblance to other skilled sports in that it demands skills be kept sharp in order to be effective. Watching a diver with 150 dives silt up a sandy bottom while a diver with 40 dives glides over it cleanly is a perfect example. Time in the water is very unequal, but learned skills make a huge difference.
Personally, I think you can learn to dive better from ANY class. Provided you have a good instructor, and this is crucial. Having a goal when you start diving is also helpful. If you get certified because you want to learn to cave dive, your path is set, and you can be very efficient in getting to your goal. Most people seem not to start that way though, and the path can be meandering and unclear.
What is it you want from your diving? If you just want to fall in the ocean from a boat, then perhaps advanced training will have diminishing returns. If you wish to do technical diving, or diving in challenging environments, then advanced training may pay you back multi-fold. However, I think it's important to train, then perform. If you're in class all the time, when do you absorb and apply? And when do you test yourself?