That's Tenets vs Doctrines, in a sense.
(Although Doctrines can impact gameplay in a major way, but they do seem of less importance than tenets)
While the UI emphasizes them a lot less, I'd argue that doctrines tend to have more gameplay effect than most tenets, currently. For instance, monogamy/polygamy/concubinage and gender-dominance both probably have more consistent effects than any tenet* in the game, at the moment. Even Communion only becomes especially significant when combined with the Head of Faith doctrine.
Most of the tenets are largely flavor-focused with small mechanical effects. For instance, Esotericism (one of my favorites, and one of the better ones of the "minor" category). It adds a virtue (wiseman) and a way to get it fairly consistently. That's a nice, consistent bonus (gaining a virtue gives you +10 opinion and +1 piety/month, which is quite useful), but it's hardly something that makes the game feel different. And a lot of the doctrines are significantly worse in that respect. By comparison, "I can only land men, and default to male preference inheritance" vs. "Both sexes are treated equally for those purposes" is a tangible effect that makes gameplay significantly different in several, obvious ways.
Even the Asatru, with their new, unique "Great Blot" tent, are basically just "Human Sacrifice with the ability to throw an extra special feast every once and a while (that is currently bugged, but that's a side issue)."
I believe that the idea was that the doctrines would define most things, and that the Tenets would make the religions feel more unique and flavorful. Which is why it's a problem that most tenets don't really feel unique and flavorful (although the lack of flavor/uniqueness is a complaint about many aspects of the game), but I suspect adding more tenets per religion would make that problem even worse.
*The only possible exceptions would be the Warmonger and Pursuit of Power tenets, which unlock the ludicrously cheap and overpowered Conquest CB, but that's an issue for a different thread. It's no coincidence that Pursuit of Power in particular tends to be the min-maxers bread and butter.