If NPCs can have 7 Intelligence, and PCs can't, then you have an in-world indicator of out-of-game status. Your world is necessarily less believable, and less internally consistent, than if you didn't have that.
Whether or not you (or your table) care about the world being consistent and believable is a matter of personal standards. But regardless of where you stand on that, some people do care quite a bit about it. Whether a designer should cater to players who care, or the ones who don't care, is a decision for them to make.