Yes I understand how the debt purchase places more money into circulation, but that will happen whether or not the Fed buys old or new debt, as the level of debt will remain the same. One can't uncouple the actions of the Fed and the Treasury here from the standpoint of an investor bank. The Treasury is scheduled to issue another $X billion in 10 yr notes two weeks from today no matter what the Fed does when it starts QE2 buys tomorrow. After that Treasury issue two weeks from now the level of debt will be exactly the same whether the Fed bought $600B from GS, or directly from the Treasury.
As I understand it the only reason the Fed buys debt via, say Goldman Sachs, and doesn't buy directly from the Treasury is investors would very reasonably suspect the Treasury can't find real 3rd party buyers (who can't print there own money) for its debt any more and flee the market.