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I see a legitimate need for rebreather and cavern and wreck if you're penetrating to the point of not being able to see daylight or your point of entry/exit.
Deep diver, not so much. This one I find particularly frustrating. Why doesn't AOW certify you to the max rec limit? So PADI can milk another hundred or so dollars out of you?
Are there any dive resorts that actually ask if you've done this before allowing you to dive to 130'.
Are you going to get nailed by the PADI police if your logs show you've gone deeper than 130' without having the card?
Did the guy that broke the Guinness Book of World records have the "max depth of any human" specialty course?
Your question seems to be whether or not specialties are required and whether or not they are worth the money.
With respect to diving instruction, the answer is more nuanced than "milking another hundred dollars" out of the client. I'm sure there are shops that do that, but the system is not specifically set up to do that, although it does facilitate it.
"Adventure dives" are there because there is a market for divers who want to try something (maybe once) or are considering buying a bit of gear and are interested in getting better informed. If you don't want or need this, then don't follow them. For some people it's fun. For others it's a waste of money. You're in control here, nobody is forcing you to take an adventure dive.
"Specialties" are there because there is a market for divers who want to try something more than once, perhaps because they are seriously considering buying a bit of gear and want to have some basic proficiency using it before they buy. An example of this is a drysuit. In my day, there was no drysuit course. My "instruction" consisted of the shop saying to me, "dive shallow until you have it under control". For some people such a specialty has value, for others, it does not. Again, your choice. A second reason people take specialties is to "fix" skills problems that they are having. In a perfect world this wouldn't happen but in the world we live in, some divers take specialties (like PPB, navigation or a number of others) in order to overcome specific issues they are having. Again, if this doesn't apply to you, then don't spend money on it. There are other motivations as well but I' won't list them here. The point of this is to show you that not everyone has the same needs or desires as you do so the system is made in order to cover the wide range of reasons why people might seek additional training. Your needs are not the only needs in the entire population of divers.
As for specialties that are "required". You might need an EANx cert to get Nitrox fills and frankly it would be unwise to dive with Nitrox without some basic training. For renting certain bits of gear the shop might want to see that you have some basic skill with it. After all they would like to have their gear back in one piece and preferably YOU back in one piece as well. This is just common sense.
As for the deep speciality, what you have to understand is that PADI is a world wide organisation. Where you live, the difference between diving to 30m and 40m might only be that you're a little more narced and you have a shorter NDL. Where *I* live a dive to 30m, especially for beginners can be very intimidating and very challenging. There's a huge difference diving to 30m if the water is warm and clear, your gear is light and you can look up and see the boat. In our context, you're gear is heavier, it's *pitch* black at 30m and the water temperatures are uncomfortably cold. The narc can worse, navigation is more complicated, the bottom is mud and even keeping together can be a challenge in and of itself.
In that context, diving to 40m with an inexperienced student would be down right stupid. Training our local divers to 30m is dangerous enough as it is and we definitely want our students to have some post cert experience in the 25-30m range before even considering taking them to 40m.
So once again, you're looking at your own situation and projecting your own diving and needs/desires as a person over the entire diving industry and suggesting that they must be trying to milk students' wallets because it's organized like that. That irritates me because whenever I hear someone saying this, it's invariably due to them seeing themselves as the model of the only scuba diver in the world for which the course should be designed.
R..