Sure their exact purpose is tied to whatever happens at the end of the story, but that isn't stopping from say one of them (Pluton/Uranus) to be abused for all the wrong purposes.
Yeah, with Caribou he could spill the beans after the antagonists are defeated, potentially to CP0 or that one guy he knew at FI.
When things are said and done Caribou's sole purpose in the story as a character is him carrying that info. about Poseidon. You can't help but think that it would be weird that Oda painstakingly places this guy in Wano through a dedicated cover story and has him nothing to do with Kaido & BM's nefarious plans regarding the ancient weapons, especially when that plotline's been absent for years.