What are so bad about Mutts, or "Mongrels"?

Once again, all those spazzy dog question answeres and dog owners, please dont get upset over nothing. I was just wondering, because when I say like "I want to adopt a dog at my local shelter or petfinder.com what mixed breed do you suggest?...people on here are like "why? that means you are going to get a diseased, mutt...well maybe I want a mutt, because people like you won't buy them! So is buying a mixed breed bad? Not that I' am going to breed two different breeds, (I do not like bad BYBs or puppy mills and I do not breed dogs, just save them from shelters) I' am just wanting to get a mix, because it is harder to save a purebred