The advantage is with the clean-up. First strike takes out some amount of launch vehicles. ABMs have a prayer against x-y warheads, where y is the attrition fro

The advantage is with the clean-up. First strike takes out some amount of launch vehicles. ABMs have a prayer against x-y warheads, where y is the attrition from the first strike (which is nonzero unless you're dealing with a launch-on-warning opponent, which neither the US nor Russia nor China is). Few powers are really ready to perform launch-on-warning. The consequences for a false positive are too dire to consider, and we all also have a survivable second strike capability (with the possible exception of China, whose submarines are boiled crap run with an Evinrude outboard). And in my estimation, even China would ride out the first wave.