Does it really matter what the credit rating is when the Fed is buying the debt?
Your premise is incorrect, because the fed is not buying the debt, as you put it. The "bank" that the feds negotiate with in order to "raise the debt ceiling" comes from selling Treasury Bonds to investors, which includes other countries such as China. We can sell loads of Treasury Bonds at ultra-low interest rates, thanks to the fact that we've never defaulted on them throughout our 235 year history.
Break the rules once, and it will never be the same. Lower the credit rating, and from now on we'll never get the interest rates so low, and buyers of our bonds will have leverage over us like they've never had before.
It's like any other traded security. The highest rated ones are more "skewed in favor of the House" than the rest. You get very little yield from the top rated bonds, but you get the security of their rating. Conversely, you get much better yield on the junk bonds, but the risk is present that they never pay up. It's not something we would want to see in our lifetimes, to have the U.S. Treasury Bond rated at Junk status.
It would translate to higher interests for every U.S. citizen, because the source of every loan out there is backed by the rating of the government. See how well the economy goes when mortgages go back up over 10%, and car loans are closer to 15%. Not to mention the international ripples of a U.S. default....