There are many many pedigrees in rescue too... Not to mention many breed SPECIFIC rescues....
So, theoretically - you shouldn't have to bother buying one of those from a breeder either.
In fact - while we're on the moral soap box - surely people are doing a dis-service to ALL dogs by buying from breeders FULL STOP while there are so many dogs in kennels - pedigree and mixed.
Although I mostly agree with you, I think it's totally different.
For mutts, whether you get it from a breeder or a shelter, it's the same - in both cases, you don't know what you're going to get, and in both cases, as 99% of breeders who breed mutts don't do health testing on parents, you're just as likely to have a dog with a genetical disease.
For purebred dogs though, it's not the same. Sure, compare your shelter dog with a BYB puppy, it will be the same as mutts - you don't know what you're going to get, whether health or comportement-wise. BUT if you get your purebred puppy from a reputable breeder, you will be 90% sure of what you're going to get - THAT is the difference between mutts and purebred dogs.
Overall though, I agree with you, which is why I despise breeding when it's not just to improve a breed (ie, breeding maybe once every two years that perfect bitch with that perfect male). The problem though, is that for people who are looking for something specific in a breed, pretty often your shelter purebred won't cut it, because he probably came from a puppy mill or a BYB, and such won't be a good specimen of the breed.
I guess the problem really is for people to stop focussing on one aspect of a breed, and maybe open up a bit to a dog that might not be a 100% match, but maybe just a 80% one. In all honesty though, no matter how I would rather get my next dogs from a shelter or rescue, if I have children it will limit my choices a whole lot - and I'm sure it's one of the problems with rescues and shelters, that if you didn't bring up the dog yourself, you'll never know what happened in their past, and it's a risk lots are not willing to take with children.