I think that the reason many people on here, including myself, refer to bully-type dogs of unknown lineage as mutts is so that not all dogs with muscles and a big head are called a Pit Bull, even by their owners. The general public would be shocked to see what a true Pit Bull looks like. They consider anything and everything vicious to be a Pit Bull, so those who know better simply try and educate when we can that technically, they aren't Pit Bulls, they're mutts. How many of the dog attacks that you see in the media come from registered APBTs? I'd guess almost none. That's why I think the distinction is important. With a dog of unknown lineage temperament will always be more of a gamble.
See, the issue is that someone that labels a dog as a Lab isn't giving that dog a negative stereotype. But if someone labeled the same dog as a Pit Bull, the dog would automatically receive much different treatment by many people.
So I understand that most people DON'T understand what a Pit Bull is, and they consider the term to be much more vague than I do. While I'm not going to tear someone's head off or demonize an entire organization for using the term, I will try and individually educate each person that I hear using it. I make a point when people ask me what breed my dog is, or if she's a Pit Bull, to tell them yes, the general public would consider her a Pit Bull, but in reality I have no idea what breeds she has in her. That usually sparks a conversation that allows me to explain further.