Everyone needs to realize we are always talking about a team of 4-5 DPS. Not everyone is competing for every spot. SAM has the highest built-in DPS, but this is a good thing for the supporter DPS classes with strong buffs, because SAMs are the best target for those buffs, the biggest force multipliers. So a DRG with a SAM is, for example, a better combo than a DRG with a MCH because the added DPS by the dragoons buffs are just better.

And so on. Basically, SAM fits in a specific spot in setups as the "golden child" DPS that you put your buffs on, but there is really only ONE such spot, and its existence is due to the fact that other DPS can buff the golden child so well.