Two additional brigades = Increased logistical footprint
Increased logistical footprint = Increased dependence on Pakistan
Increased dependence on Pakistan = Decreased diplomatic and military leverage
... and the US needs every bit of leverage it has there to get anything done under the current paradigm.
So the only real good choices are to either work creatively with the forces already there, or break out the present paradigm. Incidentally the second choice cannot be achieved with two brigades.
IMHO ex-SecDef Rumsfeld understood the problem facing the US in Afghanistan just fine. No major US leader is proposing a real, dramatic change to the basic parameters his team had initially worked with. Minor alterations won't get you anywhere better, but some of the proposed small alterations will definitely take you to a worse position. - Just my $0.02, etc. etc.