The fact that the Cullens don't sleep is 10 times more fascinating than the fact that they won't die. I don't think about what they do with their extra 100s of years nearly as much as I think about what they're up to all night. When the world around them goes quiet and they're left behind, wide awake, a myriad of possibilities arises. I dont care about their super long life, I care about their double life. The immortality that comes from their constant conciousness is way more interesting than escaping death.