From the official Iraq War After Action Report:

In this After Action Report from the Iraq War, now being conducted, the Army, finds that the deployment to the National Training Center (NTC) at Fort Irwin (among other places) was perfectly sufficient to "produce a seasoned fighting force that was trained and ready to fight and win on ANY battlefield":

Not only that, but after some problems with water supplies keeping up with the front lines, the recommended solution to that was to become completely self contained, "as though we were going to the National Training Center". A function that would NOT be present when simply rolling down to the PCMS from Fort Carson, where those supplies would be availible.