The problem I see with Bastion is that we are already really durable against stuff that isn't hostile to us. Going off what we've seen with Shenhua (who is incapable of restraining herself and thus the cultivator equivalent of what we'd want to protect ourselves against) a Cyan can handle her attention poorly. Scaled down that means that this would help us handle Prism+ inimical spirits.
I'm not expecting to encounter Prism+ spirits any time soon. Of course we did unexpectedly encounter Grydja, so maybe its a good idea? Still seems really specific
I don't think that comparison is fair.
For starter, Shenhua being "uncapable of restraining herself" is hardly a good point of reference in order to calculate what level of environmental hazard we can withstand.
Ling Qi knelt before her as a fresh green and didn't suffer phisical damage, but that's just because Shenhua didn't manifest her domain like how she did with Renxiang.
We are NOT surviving any harmful Prism-level effects, Bastion or not. Even an active Cyan would be a high order, due to the use of Shen.
Second, the whole point of MoSS is that spirits don't need to be actively malevolent or hostile to inflict some serious damage.
Bastion's purpose is to lessen that harm on us and our companions long enough to explore hazardous areas and establish contact with the responsible spirit.
Not just to
endure the damage, but to reduce it all together. This is going to be highly useful when dealing with peak green and cyan spirits. That's the kind of threat we are expected to handle.
Look at what happened with Black Sky Yearning. Even leaving aside the dream trap, her mere presence affected the weather and made advancing difficult, even for a group of well-equipped greens. After we won our bet with her, that hardship was still present, tough lessened.
Heck, that's actually in Bastion's text. It negates the
traits of higher spirits.
Raising the Bastion: In the harsh south, under the gaze of Bleak Skies yearning, you received a taste of the casual cruelty which spirits unbound and uncaring of man may display. In the lessons of the Bastion's Melody, you find the lessons needed to mitigate their harm, and even bring understanding of their doings to spirits most inhuman. Alters the Bastion's Melody Art, giving a chance to negate damaging environmental traits of higher spirits. Advances the Community and Protection projects by one step. Unlocks one project. (0/5)
Bastion would allow us to more safely deal with those kind of spirits as well as the underground toxic environment.
That's why it's a good fit for the Cathedral plus Diviner combo.