Whats so bad about runt puppy's?

I own a beagle that is a runt and she is as healthy as any other dog, many people on here have said that you shouldn't buy runts cuz they have so many problems but if no one buys the runts then there gonna be killed, personally i think runts are just as good if not better than normal dogs. Runts have just as much problems as any other dog, but why do people keep saying that its so bad to have runts cuz its not and its pissing me off!!