Ron Paul certainly deserves criticism on many issues. So does Obama. That has been Glenn Greenwald's point. You can't avoid criticism of Obama by changing the subject. A more interesting article would have been "Obama's useful idiots on the left", since, as Greenwald notes, Obama has relentlessly attacked civil rights, entrenched executive secrecy and authoritarianism, and used war to further the economic misery of most Americans (not to mention the misery and demise of the people who happen to live in his "theatres").
I would go further and note that Obama has not been even center-left on almost all social and other domestic issues. The balance of good and bad in a candidate is one that must be weighed by each of us, but Paul's "positives" are transformative and Obama's are tepid (at best). There is no shortage of negatives from either of them.
The obvious worry of the Obamacrats is that they would actually have to answer to Ron Paul as the Republican nominee instead of one of the mainstream candidates, who, since Obama is already so far right, are easily dismissed as extremists in their efforts to outdo him.
Of course, if we had an actual democracy here in the U S of A, we could talk about Rocky Anderson, running in the Justice Party, who deserves our votes more than either Paul or Obama or anyone else spewed up for us by the two Wall St parties.