Why is it important to have an internally consistent mythology? Why do I harp on “Lost” for mixing up multiple forms of mumbo-jumbo? Characters need a world in which they can conduct their own search for themselves and their own meaning. They need to have something stable but flexible to push against, to test themselves against. Otherwise they just become pieces of furniture which you move around; they’re wooden; they’re not believable. We don’t root for them. We don’t care because we can’t transpose ourselves into the characters’ situations because in a universe without rules, we don’t know HOW to succeed, if indeed we should come to want to succeed.
The television show “True Blood,” for example, mixes up vampires, werewolves, psychics, god only knows what else. One character, finding out her boyfriend was a vampire, asked, “There are VAMPIRES?! What ELSE is there?” Well, in a stupid universe with all kinds of made-up junk, her boyfriend could well have answered, “There are 150,000 species of insects, cars that talk, balloons that make you pregnant with quintuplets, a chemical solution that makes fat professors skinny and cuddly green aliens.
I could believe in vampires. I can’t believe in the world of “True Blood” with a jumble of scary nonsense.
Contrast, if you will, the Harry Potter stories which created a consistent, believable world. It’s all warlocks and though their rules might be complicated, any character can come to learn the rules and make her/his way in that world.