The OP failed to mention something: guided missiles work very well when combined with other air units. Let's say you have an amphibious invasion force, with battleships, transports full of marines, carriers and fighters, and subs or missile cruisers armed with missiles. The fighters can redline the defenders, and the cruise missiles will destroy them. The OP mentions it takes many GM's to take out a mech infantry. Well, not if that mech infantry is already weakened by a bomber. Your bombers can bring the defenders down to the minimu (is it 50%?) and then the GM's will destroy the defenders. Also, the OP probably used them against bunkers or something, because when I've used them I noticed they were quite powerful, usually completely destroying a unit if it was at 60% health or lower. In any case, I think GM's in the game have a similar use to what they do in real life - they are defensive units, and most useful in naval combat. You don't get nukes in every game. If you have nukes, tactical nukes are probably much better for defence than GM's. However, if you don't have nukes, GM's in your coastal cities/missile cruisers will help defend your land against an enemy invasion.