in an all out war, it certainly is a numbers game. There just hasn't been all out war in decades, but that doesn't mean one isn't around the corner. Being prepared is better than not.
What constitutes an all-out war in the modern era? If it includes nukes, numbers have little value. Even pulling nukes out of the equation, modern armaments make the idea of overwhelming the US with bodies storming the shores/borders an almost certainly futile strategy.