No matter how well-trained, a dog can lunge or there can be some crisis or emergency where the collar needs to hold. I agree with the others . . . for tags, that's one thing. But I wouldn't trust a plastic buckle to hold Sarama at 42 lbs, let alone a large dog. And even if Sarama had perfect walking manners (which I confess she doesn't) that doesn't mean that something unforeseen isn't going to happen that will require holding onto that collar for dear life. A squirrel leaps out under her feet and darts into traffic. Another dog charges out and attacks, and I need to hold her collar while holding off the other dog with my other hand (this has actually happened). In fact, that's why even her "tag collar" is leather and buckled. I might need to grab it for some reason and that reason might well be life or death.
Neither Breeze nor I believe that all 100lb+ dogs need prong collars, or even back-up, necessarily. . . Just that you should take into account the size and strength of your dog (whether it's 40 or 130lbs) and buy and use appropriate equipment on them. It's not for all the times YOU can control YOUR dog. It's for the one time you can't, or the situation is totally out of your control, or it could save your dog's life. When you have solid training in most situations I think that's great, but your collar and lead should be your back-up if training doesn't work. If you have a 1-inch plastic buckle collar on most dogs over 60lbs, that's not adequate back-up.
It really is just common sense.