By means of a US first strike about 99%+ of Russian nuclear forces would be taken out. So Bush Jr. needs ABMs to take care of what remains. And in any event what really matters here is the perception. Namely, the United States Government believes that with the deployment of a facially successful first strike capability, they can move beyond deterrence and into "compellence."
In other words, with an apparent first strike capability, the USG can compel Russia to do its bidding during a crisis. The classic case in point here was the Cuban Missile Crisis where the Soviet Union knew the USG could strike first and get away with it. Hence they capitulated.
This has been analyzed ad nauseam in the professional literature. But especially by one of Harvard's premier warmongers in chief, Thomas Schelling --winner of the Nobel Prize in Economics granted by the Bank of Sweden-- who developed the term "compellence" and distinguished it from "deterrence."
The USG is breaking out of a "deterrence" posture and moving into a "compellence" posture. Easier to rule the world that way. Henceforth the USG will be able to compel even nuclear-armed adversaries to do its bidding in a crisis or otherwise.