That tinkertoy approach actually costs you more in the long run. Most missions we have now don't need assembly.
We don't have any missions now
that require an HLV. In the long run it is
cheaper to tinkertoy missions together. Economy of scale on MLV's out paces launching one or two HLV's per year pretty much any way you look at it.
Just for example, how much mass could you put in orbit aboard Atlas 5's or Falcon 9's compared to the cost of one launch of SLS? Assuming SLS ever gets built?
Carolyn goes on to say in the same post that "Planetary scientists want cheap rockets
". Which completely conflicts with her assertion that she wants SLS sized missions.
Heck, your quote from John Grunsfeld contradicts her quote about rocket coming before the mission. He's designing a mission (this super resolution earth telescope) before having a vehicle to lift it on.
And neither of them seem to understand that SLS will leave no money to develop those missions they want.