The problem with Sam "letting Brittany be weird with him in ways she couldn't with Santana" is that basically nobody except maybe Heather Morris likes that Brittany, more or less everyone is getting tired of her staying so stupid and childish, and wants her to at least start growing up, and ever since she started dating him she's pretty much back to her dimwit ways, and she's already flunked senior year once. And I know that the writers have made remarks that Brittany will progress this season, I hope they keep that word.