Perhaps you can point me to some of those times where you've "repeatedly and quite clearly stated the opposite", which I assume would be that government borrowing can in fact get too big, and perhaps give me some indication of what that limit might be. In the mean time, I'll just point out that if "control of our currency gives us leeway to enact expansionary fiscal policy", then we can choose to do that without raising the debt limit by simply printing more money to pay our debts and finance our future spending rather than borrowing more money to pay our debts and finance our future spending. Given that, perhaps you can explain to me just how a lack of Republican enthusiasm for raising the debt ceiling is our big problem rather than our crushing debt and ever-increasing deficit. Surely ratings agencies are smart enough to know we "cannot default unless we choose to" so what's the problem?
There's a lot I'm going to explain here.
First, me saying the opposite of government deficits/debt can never be too big:
It really isn't.
Excessive deficits can create other problems for our country, like inflation. (although not in our current situation) You could say that spending is inflation constrained, and inflation CAN have a revenue component to it, but they are nowhere close to the same thing. That's one of the big reasons to spend big now and raise taxes for the future.
That is almost word for word what I'm saying here, and that was a year ago. See?
Secondly, we could most certainly print money instead of issuing debt. Why do you ask? As for why lack of Republican enthusiasm is the problem is that basically all economists agree that the US should continue to deficit spend in our current situation. The Republicans are trying to stop that, therefore they are a problem. Pretty simple, really.
Finally, the ratings agencies specifically mentioned a lack of faith in the US political process as one of the biggest reasons for the downgrade... ie: they know as well as we do that it is by definition impossible for the US to default unless it chooses to, they stated they were worried the US might choose to. (also, government debt can lose value due to inflation, etc.)
Simply put the US's debt is nowhere close to our largest problem right now. Furthermore, austerity would only make things worse, as evidenced by pretty much every country that's tried it in this crisis. Why anyone would want to follow such abject failure is beyond me.