In the single-player campaign, the SAM Site is the Soviets' only viable defense against air units, as the Rocket Soldier is available only in skirmish and multiplayer, and the Mammoth Tank's missiles are not effective against helicopters. Even so the SAM is nowhere as lethal as the Allies' AA gun, and you'll often need many of these in order to fight off Allied Longbow raids. Conversely as an Allied player, you can take advantage of this lack of lethality and use groups of Longbows to destroy SAMs and move on to whatever they're protecting.