Risk assessments can serve as a vehicle for early buy-in, a method to secure budget and resources, a gauge for progress, a way to avoid fire-drills and set priorities for your program, a natural reporting-out format, and proof for regulators of the adequacy of your program.
Moreover, because protecting America's entire civilian electronic infrastructure is not fiscally feasible and because a ballistic missile is the most likely delivery vehicle for an EMP attack, the most prudent method to protect America is a missile defense system that could destroy a ballistic missile before it reaches U.S. airspace.