Beginning in the mid 80's reg manufactuers marketed plastic second stages as being really great as they theoretically allowed a more optimized case design and allowed the addition of features like flow vanes etc, that made the reg more user adjustable. They were also marketed as being lighter and more comfortabel to use.
In fact, I have never seen the performance increase that was supposed to come form the optimized case deisgn as the older metal case designs were already quite good. And as far as adjustable flow vanes go, most divers dive with them more or less in the middle which resuts in a venturi effect nearly identical to what you got with a metal cased second stage anyway.
Weight is a potential benefit, but plastic second stages have to have a much larger and bulkier case to accommodate the same size diaphragm so you have to accept greater size or a smaller diaphragm with less mechanical advantage. And in terms of comfort, hose lenght and mouthpiece design have far more to do with comfort or jaw fatigue than the minimal difference in in-water weights between plastic and metal second stages.
Plus in addition to being far more prone to scuffing, cracking, and general wear than a metal second stage, plastic second stages have poor heat transfer traits which makes them more prone to freezing in cold water and due to the same lack of heat transfer, they also produce less condensation which results in plastic regs causing dry mouth on long dives.
So the only real winners were the equipment manufacturers who went from using a comparatively expensive to produce chrome plated brass case to a plastic case that cost about $3.00 per unit to produce. I would have perhaps been more appreciative of this except the cost to the consumer for these cheaper (in every sense of the word) second stages actually increased as they priced them according to marketing hype rather than on their real value to the diver.
Essentially the dive industry screwed divers in the 80's and 90's and, believing the marketing hype, most divers thanked them for it. The ill informed will for the most part not be caught dead on a dive boat with an "old" metal cased second stage.
I have bought numerous Scubapro Adjustable and Balanced Adjustable second stages on e-bay and while condition has varied with some being pristine and others being extremely used with copper or brass showing inside where the chrome has corroded away, I have yet to find one that could not be returned to fully serviceable condition relatively easily. Most, when upgraded with the current S-wing poppet and balance chamber used in the G250, breathe as well as or better than a G250 or S600 while offerring more durabilty and less dry mouth.
