Do WE, the playerbase, need them? No not really, we'll find a way to manage if they take a hike of indeterminate length. Do the devs need them for marketing, pr, and other brand recognition reasons? You bet they do, which is why they will continue to drag them through the story kicking and screaming no matter how many times they "die". And even if they let one of them stay dead, you can be sure that they will find a way to milk that death and remind you of it at every opportunity to keep you emotionally hooked. Is this an overly cynical take? Maybe, but to be honest it's the only one that makes sense to me given the facts that I have available to me.