I walked my dogs on a harness all the time when they were younger. I still do if I think there is a chance they will pull.
I just want to put in one word about training with only a flat collar. My current trainer challenged me to do this with Rylie. She does not allow any physical or verbal corrections (this includes collar popping or dragging the dog where you want it to be). I would say it made me a better trainer to accept her challenge because you have to have 100% attention on your dog if you are going to train with a loose leash. She would have allowed the halter but doesn't allow corrective collars. I think because I took her challenge and worked on building the focus for me there are very few things distracting enough to Rylie that he will pull on the leash. He isn't perfect though- I did find out the other day that he would like to chase possums if allowed. Max generally needs the harness if we are going to be in squirrel territory as the squirrels suck out most of his brain cells.
I would agree that there aren't enough trainers like mine out there and most who insist on a collar want some type of corrective collar. The first class I took Max to the trainer wanted him in a choke chain or martingdale and to spite her (I'm sure) Max was an absolute brat in his harness - doing things he'd never done before or since like biting on my pant legs.
Mindy Tri - Feb/97
Max - Ruby - Sep/08
Rylie - B&T - June/09