My "rationale" was that if the the game was about fate then the theme of the game might be taken to demand some wild coincidences, and even if not that, then they might be appropriate. Also, good job just blasting past the idea that even those wild coincidences need to be consistent with what's already established. Also, sometimes even in somewhat more grounded fiction, a character might have incredible luck--or a wide net of contacts, or whatever--as at least a part of their shtick, and it might make sense for things to break their way, or for them to know someone here. GM introduces NPC; player does mechanically allowed game thing and says, "I went to college with him!" The GM now has the challenge of running the scene, with that change, being consistent with all the things that are established--none of which necessarily needs to be anything in the way of an explanation of how did the PC's college classmate wind out as a waterfront thug?
No, I'm saying "improbable nonsense" will be dependent on context, as well as on personal taste. One scenario might demand it, one might tolerate it, one might reject it. I'm also saying that even if the scenario kinda demands it, it's still at least very likely to need to be consistent with what has come before, what has already been established; I'm also saying that maintaining that consistency is the creative challenge a GM who allows such things is accepting.so you are basically establishing a scenario in which it works by default and then try to use that as proof why it cannot ever be improbable nonsense?
That does work..
As it happens, I personally do not prefer games that specifically allow the players to add such things in play, though as GM I very much enjoy asking them for things I can use, that I then have to make fit.