Pets are pretty dang expensive. Potentially thousands of dollars a year in vet bills, and hundreds in food, just to start. I had friends when I was a kid who had pets die because their parents couldn't afford vet bills (not very responsible to have pets when you can't afford to take care of them, but you can't tell someone not to have a pet any more than you can tell them not to have a kid). Among the people I've known, pets are quite probably the biggest nonhuman financial stressor - bigger than cars.
Shelter pets are not for everyone either, although avoiding pet store puppymill pets is obviously desirable.
Most shelter animals are just fine. I got two cats (bonded pair) from a shelter in August 2013 and they've been a dream. Shelter life isn't easy for animals, but it doesn't typically break them (a few handle it badly, but most are psychologically normal after a couple weeks in a new home).
You're taking a bet with any kind of new pet, but you're probably getting a healthier animal, on average, with a mutt from a shelter than if you get a breed animal, even from a "responsible" (i.e. not puppy-mill) breeder. The better shelters don't do that much psychiatric damage, and most of it's not long-term. This idea that shelter animals are all psychiatric/behavioral basketcases is both untrue and very damaging.
I prefer no-kill shelters for finding pets. Part of it is selfishness (I don't like feeling like not choosing an animal will condemn it to death) and some of it is ideology (I don't agree that it's "euthanasia" to kill healthy feral animals; who are we to decide that they'd be better off dead?) but I also know that NK shelters (which source from "kill"/open-admission shelters, so you're still saving a life) are staffed by people who are experts (at least, relative to me) in selecting adoptable, healthy animals who are unlikely to have behavioral problems.
Shelter pets are not for everyone either, although avoiding pet store puppymill pets is obviously desirable.