Besides, Essentially, adding tremendous weight and expense you mean? :-) The old anti-SDI crowd always made these silly arguments that the boosters could be 'hardened' against boost-phase attacks. Maybe if resources were infinite. Most of the major threats we see today operate on a shoe-string, however, from the Iranians, Pakistanis, North Koreans... Even the Chinese.
Only the Russians might be able to pull off something like that hardening as surmised.
As for the warhead, the system would not have any direct effect on that, as
Pat Shanahan of Boeing made it clear that this is is a boost-phase defense: "Proving the capability of this laser to operate at lethal levels of power and duration moves the system a major step closer to becoming a vital component of the nation's boost phase defense against a ballistic missile threat.
Boost-phase attack is the most attractive because it is the most vulnerable area of the missile system. It is a big flaring IR (InfraRed) signature, for one. Easily targetted. Second, it is extrordinarily vulnerable because it has to be kept thin and light just to get off the ground...or to deliver a payload worth the expense. Even the solid rocket boosters (such as China's brand spanking new "East Wind" DF-31) are at risk of destruction by an attack from this system. Their guidance rockets are typically liquid...
Whatever happened to the "Rods from God" program?
I was talking about mirror polishing the outer skin, rather then adding a big mirror. That would not add weight.