I think the drafts are especially useful for something like a BSOD or a power failure where I'm actually in the middle of writing an answer when it disappears. But if I've gotten distracted, if I'm on another tab and accidentally (or intentionally) hit the browser's close button without thinking about it, I may not even remember 5 minutes from now which post I was writing an answer to. I may not even remember that I was writing an answer! This is especially true at work when I'm writing in little spurts at a time and have to shift my attention constantly.
I think the warning should stay. The two features are complementary. The warning is a reminder that I never finished answering the question; then, I can actually close it anyway, secure in the knowledge that I'll (probably) be able to get it back later.