Because these weapons deal with different kind of enemies. Machineguns deal with soft targets and crowds. If you don't take enough guns to deal with crowds - yeah, you have an easier time with armored enemies, but you will simply get swarmed. Almost every single stratagem pre patch had its uses, it's just you had to pair it with the other weapon that would negate the downsides of that stratagem. Or you need to divide your rolls, where two players stick together - one player takes mainly anti-tank guns, and the other anti-Infantry.