I can’t really speak on EU PF, but a lot of the groups I see on Aether that have locked party slots have either done it for speed/parse runs (so they want the meta, which SAM is not a part of due to its lack of utility), or they locked it to “role”, and a MNK already joined in the MNK/SAM spot (they do this for DPS to prevent multiple DPS from needing on role gear, and usually do the same to RDM/SMN/BLM and BRD/MCH—NIN and DRG are lucky since they don’t share any left side pieces with other jobs). But that’s Aether.
It could be that they want the meta for supposedly fast/clean clears, but locking a PF to the meta comp will not necessarily mean you’ll have a fast/clear clear; it’s the same as when people lock the item level to 335 or 340; higher ilvl doesn’t mean better players, and the same applies to the meta comp unless the party leader “asks a little birdie” about the people that join their group and their performance.
I know it’s probably not the best solution, but you are always more than welcome to create your own PFs without any role locks. Sadly, the party leaders of PF parties are entitled to set whatever requirements they want for their PFs, and there isn’t anything that can be done about that. (The “BL. :’)” attitude however, is unnecessary.)
With regards to SAM and its lack of utility: when SAM was first announced, the developers said outright that it would be a selfish DPS with no raid-wide/party utility outside of its high damage. Which, to be frank, isn’t really that high since MNK can keep up with a SAM, and MNK offers raid-utility (Brotherhood/Mantra). MNK was in the same boat in HW, which posed a lot of issues because raid comps did not favor MNK due to it’s lack of contribution to a raid as a whole (same with BLM, and the fact that BLM was difficult to optimize in any fight that required a lot of movement). MCH does have utility, and it is still part of the meta, though it will sometimes be subbed out for SMN because SMN is just monstrous right now in terms of DPS, and it can Raise party members (RDM is usually only used for prog because its personal DPS pales in comparison with SMN now, and BLM is just in a very bad spot in general right now). It (MCH) just doesn’t offer the same amounts of support a BRD does, but BRD has always been the king of support when it comes to raid comps.
Honestly, the only way to really break the “meta” would be to do something with NIN (NIN isn’t part of the meta solely because of Trick Attack, but because of it’s aggro utilities Shadewalker/Smokescreen) and to do something to break the DRG/BRD/MCH meta, which would involve addressing the piercing resistance down debuff.
Since SB did not introduce any new job that gave piercing resistance down (and did not just let BRD/MCH apply it themselves), BRD/MCH just shrugged their shoulders at the other jobs and continued to cling to DRG, because their damage is so directly affected by a DRG’s Disembowel (and BRD benefits A LOT from Battle Litany, but not as much as from Disembowel). To give a bit of insight, I main, and raid with, BRD, and with just Disembowel alone, I consistently do 400~500 more DPS for the entirety of the fight than if I don’t have Disembowel. Combine DRG’s Battle Litany with a SCH’s Chain Strategem, and my opener can easily burst for 8k to 9k DPS; without, it can sometimes break 7k if I get lucky with Pitch Perfect procs/if their’s an AST and they Expanded a Balance or gave me a Spear. And that’s fairly significant.
NIN has consistently been in the meta since it was launched in 2.4, and the job has never received any sort of significant nerfs to it. I really do like the utility NIN brings, but if SE wants to discourage the physical DPS meta (which I think is one reason why they nerfed Disembowel from 10% to 5% in SB), they need to give other jobs equivalents of Shade/Smoke. But such can be really difficult, because it may destroy the identity of NIN as a whole. And, not to speak ill here or anything, the developers have proven that they struggle with true job balance several times in the history of this game (see: BRD/MCH buff wars in HW, AST in general pre-3.4 and post-3.4).
I believe that the developers are planning to address a lot of the issues surrounding BLM right now, but I think the purpose of jobs like BLM and SAM are that their personal DPS serves as the “utility” they bring. But, unfortunately, a lot of groups much prefer the types of utility from jobs like NIN and BRD/MCH (hence why those three are meta), and the buffs from NIN and DRG (so, again, meta). And it’s fine if the devs want BLM/SAM to just be these godly, monstrous DPS machines, but, as it stands now, they really aren’t. Like I stated earlier, a skilled MNK can easily keep up with an equally-skilled SAM (there isn’t a terrible difference in the two’s personal DPS), and SMN just stomps all over BLM in terms of damage AND utility right now.