Hmmm.@Wibble
The GUE world is not black and white, too. GUE does not promote their way of diving as one-size-fit-all. GUE is not as strict as you always tell us.
You are always talking about GUE and DIR without having any clue about it. Why don't you start reading e.g. the in depth GUE blog? There you can see, that GUE is fully aware of the different ways to approach diving.
At latest when attending to a Cave 2 class you will be tought that you have to be a "thinking diver". And this also means, that there usually is not only a single way to face a problem. One of the main messages is, that if you want to dive a special environment, take the right tool for the job. This includes sidemount and even sidemount rebreathers (although GUE does not teach SM rebreather classes up to now, and maybe never will).
At the early stages of technical training, GUE is very rigid. They may well take a chill pill in their most advanced courses -- Cave2 is advanced -- but the system stands.
Sidemount requires Cave2
Sidemount's relegated to a special tool for specific tasks. Outside of GUE it's a perfectly good general purpose tool for all kinds of diving.https://www.gue.com/diver-training/explore-gue-courses/cave/cave-sidemount:The GUE Cave Sidemount provides an opportunity for experienced GUE Cave 2 divers to familiarize themselves with side mount equipment configuration. A sidemount configuration provides an opportunity to experience, enjoy and explore areas that are too small for the standard GUE backmount configuration.
There's a ton of other perfectly acceptable techniques used safely across the world by many divers. Small sidemount. Solo. Lean left, rich right. Normal rebreathers. Specialist rebreathers. Deep air. Loose buddies, solo but together. Independent diving. Must have standardisation.
The standard riposte is to use the "you need to be in the system to fully appreciate it". Non-GUE people have the impression that it's a very rigid system. There's now 860 responses to this thread and it's not really doing much to change my mind (which was largely made up from my own Fundies experience).