Chances are, it's already been written off. But if I owed it, I would pay it, if for no other reason than its being a legitimate debt I owed.

That would depend on whether you had character and valued your reputation or not. If you incurred the debt, you have a responsibility to pay it, whether it helps your credit or not. Its just the right thing to do.

If you can, yes. It's just the right thing to do.