In terms of the impetus for teaching through multiple agencies - there are a variety of reasons why a shop might do this. Most of my experienced is based in the tropical resort world where the major players are PADI and SSI, but this fairly simplified multi-agency concept (given that there are many more) exemplifies some of the reasons why a shop may choose to operate with multiple agencies.
Firstly - the two programs teach, fundamentally the same thing however SSI is more commonly marketed as a three day program and PADI as four. That's not always the case, but that speaks to the majority of programs in resorts. Therein lies one reason for different agencies - time involved in gaining a certification. Whether you agree or disagree with the concept here is entirely irrelevant - that's the way it is.
Secondly - as mentioned above, PADI is more expensive and it does require the students to purchase their own copy of the course material. Whilst some people may indeed never go back and read it again, I think they are missing out on material that is extremely well prepared and I strongly encourage all divers who possess a copy to go back and read the bits they might have missed. Nevertheless it adds significantly to the price of the course, and this is clearly a deciding factor for some.
Thirdly - Brand name and perception. PADI of course is the biggest, but as is clearly shown in many posts, some people don't like them and therefore seek an alternative, of which there are many: NAUI, NASE, BSAC, CMAS, whatever, people might want to choose a brand because of their individual perceptions of a particular agency. On the other hand, PADI is so huge that dive shop owners feel they might lose out if they *don't* have a PADI logo somewhere, even if their preference is to teach NAUI courses, for example.
Fourthly - Brand loyalty. Somebody who undertook entry level training with one agency is perfectly able to undertake further training with another, but many choose to stay with the brand with which they began. An SSI trained diver arriving in Thailand is more likely to look at a dual PADI/SSI store than an PADI only store, and vice versa. The same concept is true in many other walks of life - some people will only ever purchase a particular brand of motor vehicle because that's the first brand they bought, they liked it, and feel confident that purchasing the same brand again will be a good move, if not necessarily the best.
So already there's a few reasons.
In terms of finding a refresher - they're all going to do pretty much the same thing and they don't *have* to be taught under a particular agency. PADI market their "scuba review" but at some of the places I worked for we preferred to market an agency-independent "Refresher Course" to take into account the fact that our customers came from a wide variety of backgrounds.
And a final word: The OP is ripe for an agency bashing session - so let's keep it on topic and objective, please!
Cheers
C.
