I don't trial, but my understanding of the advice not to push a young dog into defense too young is not that working in defence is bad per se, it's pushing the pup too hard at too young an age risks breaking his confidence, and that is what "ruins" them and creates the reactivity or avoidance you're referring to.
I think maybe you should take some time to learn about defense drives in dogs before encouraging people to allow their puppies to defend themselves...
The great danger when working a dog in defense drive is that the same stimuli which cause defense behaviour also cause avoidance behaviour. Which of the two possible behaviours is displayed by a dog when a trigger stimulus is presented is dependant on a variety of factors, among them confidence and temperament of the dog as well as the threatener, "life" experiences of the dog, age and maturity of the dog, location (unfamiliar or home turf), distance between adversaries, and the presence of other external influences (prey, mate, puppies).
Look at ALL of those factors that have to be taken into consideration before letting a dog get into defensive drive safely.
Lots of folks on here do work their dogs in these sports though, so maybe they can speak to that better than I can. I do stand by what I said though, stress can be very good for a dog, distress not. If you can't tell the difference, err on the side of caution!
But we aren't even talking about DOGS. We are talking about puppies
. Immature animals that haven't developed fully into their drives. Putting them into an uncontrolled situation that can get out of hand very quickly.
Humans putting pups / young dogs into defense for sport work is inadvisable, and that is with the people reading the dog and specifically not trying to push it too hard... But with other dogs? YOU CAN'T CONTROL THEM!
If you leave your pup to defend itself in the free for all that is the dog park - how do you keep the dog park bullies from "pushing too hard at too young of an age"?
"Oh hi there Mr. Husky, can you please not push my puppy too hard so you don't break his confidence?" Yeah. Not gonna work.
Rough play can escalate to a full out fight in a matter of SECONDS. Especially in dog park environments. And guess what? By the time that the humans can see that they need to intervene - it is already too late. The impression of the situation has already been made on the pup.
There is absolutely NO NEED to make a puppy defend itself so it gains confidence. Dogs are not introspective. They don't sit there and think "Oh hey I don't like getting beat up. I need to learn to stick up for myself". The confidence a dog has is largely genetic and hormonal driven. They either have it at birth or they don't. Not doing anything and letting the puppy mature without being put under uncontrollable stress will result in a confident dog at maturity - if they were destined to be one in their genetic blue print. However, putting a pup into situations where it needs to defend itself before it is mature enough to do so - it's putting that blueprint through the shredder.
Get a good pup. Do nothing with it regarding defense and you have a good dog. Get a good pup and be an idiot about pushing it's defensive boundaries and you get a mess of a dog with psych problems. < I've seen that scenario play out time and time again. By people who willing push their dogs into defense and by people who accidentally let stuff happen.