As far as I am aware, SAM does more raw DPS than MCH, even when put into fights with heavy movement. And you're primarily competing for a selfish DPS spot that happens to also compete with other DPS directly. None of the selfish DPS should ever be outcompeted by support DPS, that much is a given. MCH just happens to be the worst of the selfish DPS (as it should be) but that puts it under other DPS that aren't selfish (which is bad).
