SAM out classing all other DPS

[Nominous]

That's not the point, and you can't analyse it like this for many reasons.

It is the point, and you can analyze it like that. First thing you need to define is what the conversation even is. It is a conversation about what is best, min/maxing, and balance, or else nobody should be posting opinions here, other than to vent frustrations. Second, is how the game maintains balance, and how team comps are affected by that. And their balance decisions, by and large, come through way of the 'meta' Jobs' performance in the hardest content available (as opposed to Primals, Dungeons, and Alliance Raids). There is overwhelming evidence of this mindset for 'balance' from 2.xx-3.xx patch notes and raid Q&A in Live-Letters.

That being considered, NIN IS 'meta' for any team looking to push for that sort of thing. They are objectively about as powerful as any other DPS, but with extra 'benefits', so there's really no reason to leave it out, as it makes things 'easier' for everyone involved. You can safely assume that a team will have it, just as much as you can assume a team that cares about this will have an AST. Just like RDM is a shoe-in for casters, for very similar reasons. And yes, there is something wrong with that picture. However, it's inevitably going to happen. There's always a weakest and strongest, easiest and hardest. People want to take the path of least resistance, so if it's not NIN that's 'meta', it'll be DRG. If it ain't RDM, it's BLM. If it ain't BRD it's MCH.

Anyways, even if you remove SAM/NIN from the picture, it remains true that WAR creates a situation in which only one party member (the co-tank) can benefit proper from the Slashing debuff. Therefore, if the PLD/DRK setup brings comparable DPS, and more overall utility to the party then XXX/WAR, you can then safely assume that WAR isn't going to be in the party either.

All of this to say- SAM's slashing debuff isn't taken into account because there's few situations in which you'll be providing it as proper utility. And of those situations, very, very few matter in a conversation about balance, since, in those situations, you can't always choose the most efficient party comp in the first place (creating imbalance either way, better or worse, not only for the SAM).