That's the thing though--if they can create more at any time, why bother "borrowing" in the first place?

Tying up economies of debt issuer and debt holder seems to be the reason for the US. For debt holder it’s a fairly safe way to park money but also exert some power over US if they’re big enough. Realistically what Ben Bernake is saying in that interview is that you could replace one with the other depending on circumstances but that actually threatens capital holders hence why it’s not done like that.