If you ever have a chance to see the rear rotors taken off, you will see how skinny and small these parking brake shoes really are. They aren't much. And one good run about a mile or so leaving them on by accident would probably make them pretty inefective.
I have parked on a hill before with a heavy load and the road was slick, I had to put the truck into 4 wd and set the brake. At least the parking pawl and all four wheels kept it from sliding. I don't think the emergency brake would have held the truck by it self even on dry road. And it was adjusted up pretty good.