Ahhh - the old "national debt doesn't matter" arguement again.
I've said it before and I'll say it again. Perception is reality. Sure, you might be able to prove on paper that the amount of US debt doesn't really matter. And, you might be right. However, that does nothing to stave off economic disaster.
If people think that the country is in trouble, then it is. Doesn't matter if you can prove it on paper. If investors start pulling their money out and stop buying debt- for whatever reason, be it perceived instability, rising debt, or because the moon aligned with Venus - then the country will be in trouble.
Unless you can somehow convince the American people not to pay attention to the actual debt numbers, then the size of the debt matters.