I strongly agree with TrashMan.
In fact I'd also like to point out an important gameplay incentive to having AI controlled, non-phalanx, human soldiers. It's the fact that it takes a while to move 8 units around tactically, 12 or 16 much more so. Bulking out the player's side with AI driven, fellow earth-defending soldiers means larger battles can be fought against more aliens without taking so very long or leaving the player hopelessly outnumbered.