Training shouldn't be difficult, training shouldn't be hard, training shouldn't be physically and mentally strenuous - it should be fun. I mean, I would hope at this point, with all the knowledge of positive, play based dog training that is out there in the world, that training should be fun for both dog and handler, I mean why wouldn't it be?
Training, in my mind, is learning about your dog and working to open the communication between you two. It's the difference between having a pet turtle and a pet dog - we can communicate with them, and this is AMAZING! Why wouldn't you want to teach your puppy how to communicate with you, play with you, learn how to be a happy member of your home with manners and the security of knowing what we expect. Why wouldn't you want to take your dog when they are most open to new ideas and learning and, well, teach them all about the world and how to interact with it. Why wouldn't you want to play with your puppy? Why have a dog if you don't actually want to have a relationship with them until it's time to 'do agility' (which is apparently VERY SERIOUS BUSINESS)?
I have worked at the shelter, I have seen way too many 'feral puppies' that grew up into feral adolescents, and they were not fun, they were frustrated, didn't know how to communicate what they wanted and needed - and certainly were no fun to live with. Trust me, feral puppies (6-12 months old) that have not learned to learn are not happier because they got to be 'just be a dog'. Happy dogs have healthy relationships, know how to communicate their needs, feel secure that they know what behavior will get them what they want - and enjoy playing with their owners (and, once again, I certainly hope that play and training are interchangeable).
It just makes me sad when I hear someone isn't going to train their puppy, because they're missing out on so much. I don't know why training has become such a negative thing for some people, but maybe they should take a look at their training methods and see what's there that is making training something other than just a fun game. And of course you shouldn't drill your puppy, or do crazy dangerous things - but, yes, puppies should run, they should play - and they should do it with you. Keeping your dog in a padded box until they're at that magic age that is 'old enough' to do agility is just no life for a puppy - and to me, that is exactly the opposite of 'letting your puppy be a puppy'. Puppies SHOULD be learning, all the time - they're learning something whether you're teaching them or not.
Anyway, I certainly hope there is not someone out there that would love to teach their dog a new trick every day, run around and play with their puppy and, gasp, teach them things like stays and self control - but thinks that will 'ruin' their dog. Training, communication and building a relationship with your dog is a wonderful, positive, HEALTHY thing to do. And if the dog isn't having a total blast while you're doing it, you're not doing it right.
These puppies will start training BEFORE THEY LEAVE MY HOUSE. Poor souls.