Raising the national debt level

This is starting to seem more and more like a formality. Why does Congress bother with a debt limit at all if they just raise it any time they get close to exceeding that limit? The real limit on debt is the limit on how much bonds people are willing to buy before they start wondering how they'll be paid back. There's been talk of simply canceling the debt; if the federal government can screw over GM bondholders then there is nothing to step it from screwing over its own bondholders. But I think that the more likely scenario is hyperinflation. That's probably why it's still hard to get a loan...