Because it turns a few of the election lemons into lemonaide.
Whether Rumsfield is Sec. of Defense or not is not as important as is the perception that is given to the Iraqi Government.
This reinforces the election signal that the USA won't be there to prop them up forever. The clock is ticking.
The second reason is that it will force the Democrats to fish or cut bait on all this talk of "getting along" very early in the new congress (with the confirmation hearings).
It will also take away the "Rumsfield is to blame" card from the Democrats.
It actually makes a lot of sense.