In the resorts where I was working, the AIR2 was not considered a valid alternate air source, and each diver was always required to have one alternate air source, to be donated in case of necessity. If he also has the Air2, good for him, but this does not remove the requirement of having a secondary air source.We may be splitting semantic hairs. Yes use of an air2 is a form of primary donate, but outside of SB I have not heard one person using an air2 refer to their setup as a primary donate, they typically say they are using an air2 or a bcd inflator octo. Most folks that talk primary donate as a system are tech divers, a much more limited group.
Regarding the fact that primary donate is used by tech divers, I would add that it is used by just a group of tech divers, following a specific method developed and optimised for cave diving in a specific type of caves, which are not common here in Europe.
I have read about UK cave divers, and they use different methods and no primary donate.
The same here in Italy, I did dive for more than 10 years at Capo Caccia, Sardinia, where the caverns go inside the mountain for almost 1 km, and where specific procedures are employed, totally different from the US method.
For example, frog kicking is entirely forbidden there... And the usage of dry suits is strongly discouraged at Capo Caccia.
So, it is not that tech divers are a minority (which is definitely true), it is that, looking worldwide, also among technical divers these US-style techniques are not so common... A minority of the minority, statistically irrelevant for "mainstream" resorts.
Probably very useful and highly optimized for the specific type of diving for which they were developed, but absolutely wrong in different environments or with other types of divers.
A diver should adapt his equipment, procedure and behaviour following what is optimal for the site, without anything religiously defended and assuming it is good everywhere and always. I have no problem following the procedures and safety measures required from the resort where I am going, they probably know better than me what is appropriate in their site.