> Wouldn't it make more sense for individual agencies to re-jigger their IT policies
How do you get bureaucracies to do that though?
Not by fiat from the top. The system simply doesn't work that way, the career bureaucrats have usually made a career out of subverting the orders that come from politicians which they don't like. Read Robert Gates's book about him running the DoD to see what he had to do to get some progress on some things at DoD, and then realize that most agencies are not lead by people as capable as Gates.
That's why they are hiring some people near the top. The USDS will serve as the White House's eyes and ears for agency management of IT, and the bureaucrats within the agencies can either get on board (and get performance evals listing how they helped supported "Presidential efforts") or try to get in the way (and get caught by staff at the USDS who can actually call them out on their bullshit and teach agency heads how to spot bullshit).
Cachet isn't everything in DC, but it is a lot of what you need for change to occur.