Earlier quoted context omitted.
It's up to apps to implement it. But why, do you honestly think it will get in the way anywhere besides IDEs (and maybe not even there...)? This is an honest question because I can't really think of instances where I wouldn't want something like this. I'm spoiled by Notational Velocity, which saves automatically, and by iPad apps for same. I have Coda and TextMate set to save when they lose focus so I don't lose work…
Some people actually do use a large percentage of their hard drives.
If your autosave makes you run out of disk space, you're probably "doing it wrong".