How much over pressurizing were they talking about? 10%, 20% ,cycling the tank to hydro test pressure 10,000 times, or expecting the tank to live at or near the hydro test pressure for a significant portion of its life? All of those are distictly different. Also what works for one particular brand and model of tank using a particular steel does not apply to other brands, models and steels.
Testing to destruction with one tank does not take into account the reasons why safety margins are designed into the tank in the first place - the potential for manufacturing flaws, (inclusions in the steel, etc) and degradation that may occur from a variety of operational causes. At best, divers are using the reserve so to speak, something that would not be acceptable in gas planning, but something that ironically enough seems to be perfectly ok with regard to filling tanks - just because no one has died doing it yet.
I can see this being done in the old days when all you had were large and heavy low pressure tanks where massive overfills were the only way to get the gas required. (although if memory serves, 20% was considered pretty massive back then - 50% to 60% overfills seem to be something that divers have evolved to over time and I would not automatically assume there is a 30 year record of success with that level of overfilling.) However now with the availability of high pressure tanks designed to deliver the same volume of gas at similar pressures in a comparable sized package without breathing hard in terms of exceeeding service limits, it just does not seem to me to be resopnsible to continue promoting massive overfilling of low pressure tanks.
It also begs the question why the practice continues. I suspect some of it is just institutional inertia - the old tanks are what the old divers use and they are what new divers are either referred to or observe and then gravitate to in order to fit in.
Part of it may just be ignorance that both old and new divers are not aware of the favorable numbers comparisons between the old low pressure and the newer 3442 psi tanks - reinforced by such helpful tidbits of information as "shops will not overfill high pressure tanks" (ignoring the fact that an overfill is not even needed with them).
A good example is that a daughter asks her mom why they always cut the end of the ham when they roast it. Mom says "because my mom did it that way". But when the daughter asks her grandma, she says "because my mom did it that way". When the daughter asks her great grandma, she says "I did it that way because I did not have a roaster big enough back then and had to cut the end off to get it to fit." Some traditions continue long after their practical value ceases to exist just because no one ever questions why it was done that way.
I suspect however that some of it may just be a psychological statement. "Look at me - I ignore the regs and the safety limits and massively overfill my tanks to levels that make mere mortals cringe, because I am a CAVE DIVER." It is unfortunately a cave country tradition that divers are proud of and identify with, so they keep whacking the end off the ham even though they now have a large enough roaster.
