Former National Security Adviser Robert C. McFarlane wrote ("Don't Scrap the Midgetman in Haste," Dec. 17) " . . . within 10 years the Soviet Union will also be able to attack our land-based systems from submarines armed with highly accurate ballistic missiles." If this is true, what purpose is a multibillion dollar nuclear-defense program (SDI) that is ineffective against such submarine-launched missiles or, for that matter, against cruise missiles or airplanes carrying nuclear bombs?