Does A Rating Downgrade Matter?

Ryan Avent and Massimo Calabresi don't sound worried. Felix Salmon differs:

The bottom line is that a US-double-A is a more brittle and fragile place than a US-triple-A: if and when things go wrong, they will go wrong more dramatically and drastically than if the undisputed global hegemon was always understood to have the ability and inclination to smooth things over and sort things out. If bond yields don’t rise much in the wake of a downgrade, don’t for a minute think that means the rating doesn’t matter. It just means that the rating doesn’t matter yet.