SAM out classing all other DPS

[Nominous]

This is already an issue itself, but I'm not comfortable just discarding this piece of utility because we implicitly assume that "it's always there" (which shouldn't be the case, but that's how centralising NIN really is these days).

Another issue I have is that the multiplicative argument goes both ways- if you have a very strong base dps, you get more out of some buffs.

If the conversation is about a meta in a raid setting, and it is, then NIN is assumed to be in the party. Any group that cares about such things won't run without one. It has strong enmity manipulation, which is helpful for increasing Tank DPS through minimizing enmity combos needed. Somewhat relevant with healers, in relation to Smokebomb. TA is reliable, and it's DPS is at, or higher than a good portion of the other damage dealers.

At best, if we're talking PF/DF conditions (which doesn't matter for a meta conversation), SAM being the sole Slashing provider only benefits the two Slashing Jobs that don't have the ability or requirement to provide it for themselves- PLD and DRK. Assuming 2500 sustained for both Tanks over the course of a fight, that's ~500 rDPS contribution. Swap places for a NIN, and guess what? Still 500 contributed. The takeaway being- It's not exclusive to SAM, therefore it doesn't matter. Any argument saying that SAM gives a lot to a party with Slashing, can really be translated to the same argument with NIN. Except NIN has more than just slashing, so... yeah. The only imbalance here is that WAR is the clear loser, since it'll only ever, at most, benefit it's co-tank.

[Galvuu]

snip

That's not the point, and you can't analyse it like this for many reasons.
First, you can afford to bring PLD+DRK because the dps will bring the slashing debuff. This is an advantage of such a comp (if nothing else, an advantage of versatility).
Second, we're discussing job balance and viability.
Our premise in this setting can't be "there will always be a NIN", because if there's always a NIN, that's already a balance issue, isn't it?
If the conclusion is "any serious comp must have a NIN", then clearly NIN is too strong (and it is).

In an ideal world of almost-perfect balance where NIN isn't something that every comp should have, then SAM allowing you to run PLD+DRK just like NIN does is utility. You'd say "I need a slashing debuffer here", and that would be an attractive point and raid dps gain for both these jobs. Like it was mentioned earlier, the fact that you put buff and therefore have this overlapping (and this useless) utility tool hints at how one or both are a bit too strong.
When you can ignore a 500 raid dps bonus a job brings and it's still the part of the best comp... I dunno what to say about it that I haven't already.

Lastly, from a purely statistical viewpoint, it's bad to assume independence where it's unlikely. Which would technically be the most important takeaway, even if it's inconsequential to the conclusion x)

[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).