The Flexner Report basically eliminated quackery from mainstream medicine. Before that, you'd have some dude running a med school out of his house, teaching the healing powers of snake oil. There were so many branches of medicine that you'd have docs using treatments with no basis in reality whatsoever.
That's not to say we know everything about medicine there is to know, and I have no doubt that in 20 years, we will look back on things we do today and think "I can't believe we actually did that to treat ______", and 20 years from then, we'll do the same thing. But at least what we do now has its basis in science (for the most part), and has been shown to work, and we know why 99% of the time.
As for difficulty of admissions? THat's a tough one. As a non-trad, I am having trouble getting even a sniff from some places that I would have gotten into easily if I had my act together from day one. I know I'm smart enough for Baylor or UT-Southwestern, or whatever, but at the same time, I am glad they are selective.
Does that mean that I can't be a better doctor than people who graduate from there, even if I go to DO school? Nope. Since the Flexner report has standardized medical education, my education will be whatever I make of it, wherever I go. I'm just going to have to work harder for credibility once I get out than someone who went to a big-name school.