One is this compartmentalization that Will mentions. It’s incredibly common for people who are generally intelligent to still be religious. I think some people who are really at the top of the bell curve might have vague spiritual feelings, belief in God, religious practices because of cultural value, etc. — but not the kind of literal, obviously absurd beliefs that Z was talking about. I think, however, that the vast majority of the population, including people who are above average intelligence, can compartmentalize like this. That means that the variation in religiousness is probably explained much more by their family and other circumstances than their intelligence. Therefore for most people, religiousness is not a very good proxy for rationality, so it’d be only a very minor consideration if you were to consider it fairly (though Z’s logic still does imply you should consider it that small amount).

The other reason is more practical. If we were to allow discrimination based on religious beliefs, most of what would occur isn’t this slight mark against someone’s intelligence for believing extreme religious beliefs. More often, it’d be very religious people enacting drastic discrimination against those who don’t share their beliefs. Even if some slight consideration of religion would be rational and productive, we gain a lot more than we lose as a society by banning it.