With an adolescent or younger buddy: A decent pony lets me handle any gas emergency without stressing them or relying on their reaction under that pressure. Necessary? No. I could put them through all that, and likely get gas from them if needed, without them doing any kind of good job in the process. But a pony would be a much better plan. Instead of "make sure you save your uncle on the dive today".
I understand your paternal feelings about this but I certified many junior*divers and actually same training as adults applies to them.
With divers I know and trust as very solid: No not necessary. Though it adds an extra layer of ease when we're both redundant.
With unknown divers: Mmm. Well, which of the above two cases are they closer too? Can I know that before getting on the boat or in the water with them? Will I even know after a dive when things seemed to all go well, as they seem fine but are actually totally useless if something happens? A pony is not required... as I could still likely get air from them even if they are useless, but it would give me more peace of mind that I'm not rolling the dice on their reactions.
Trust is a cultural thing. I lived long years in Finland and Denmark, where trust to other individuals are highest compare to the other countries, and these countries often rate as happiest, safest and most competitive in the world.
No matter how safe driver you are, you are endangered by other drivers, very hard to control this, either you must accept this risk or not leave home. If you feel insecure diving with unknown people, sure, you should not be diving with them, that is most effective remedy to that.
There is perhaps 1 fatality in 100 000 dives. This includes, everyone, if you are in the median, you and your buddy are average divers, that is the risk you are taking. I would say you should not base your dive plan on catastrophic failure scenarios. If you do not trust your buddy, mitigation is to talk with him before the dive and not dive if you are not comfortable.
Having a decent pony simplifies my pre-dive pairing up process and the dive itself to "am I comfortable helping you at need" instead of "am I comfortable relying on you at need for gas right now". A modest cylinder at my side is a small price to pay for that simplification and reduced worry before and during the dive.
Any extra equipment you carry will make you less streamlined, your air consumption will increase, you will build-up more co2, you will breath heavier thus more likely to get a free flow at depth. You are adding more unknowns to the equation. Potentially it could be you that will become liability.
I think a better question is:
If every rec diver was a much better diver, would that affect fatality or general safety stats and in what way?
Everyone being much better divers would clearly make a difference. Yet I can't make all divers better. I can hedge my risk when I'm not sure how good the diver I'm with is, and I can handle the minimal pony complexity as I am a better diver.
I am sure everyone in this forum will have a differing answer on what is better diver. Carrying equipment is not necessarily diver skill. Knowing to use the right tool at the right time is. When you carry a pony for every single dive, you have assumed that you could not plan the dive, you could not asses and mitigate your dive buddy's behavior/skill, you could not manage your gas, you could not asses the conditions you are in. How is it that it makes you better diver?