It does seem to be the case that a lot of threads about proposing a feature of some sort or other tend to have someone suggest "just make it a game rule." For instance, I'm pretty sure it was suggested in one of the "boats" threads, as an extreme example (and wherever you fall on the CK2 vs. CK1/3 boat issue, I think you'll agree that having the AI be able to handle both depending on which game rule you choose would be a nightmare to code).
Game rules work very well for simple things like "Can the AI use the Seduction focus?" in CK2. That's a simple binary, getting rid of it doesn't break anything, and the AI doesn't need to be adjusted (after all, it would theoretically be possible for no AI to roll the Seduction focus naturally, although the odds against that are astronomical). The more complicated things get, the more likely the different game rules are to break something. For an example, "no matrilineal marriage" seems like it should be an easy game rule, but when they implemented it, they originally neglected to adjust the marriage AI weights appropriately, such that they went for a while with the AI refusing marriage proposals under that setting because they were holding out for a matrilineal marriage they couldn't get (proposals would get a penalty "------prefers a matrilineal marriage"). And the more game rules you add (and the more complex they get), the more likely you miss something like that (in some combination).