I've heard it when breeding lizards (leopard geckos, bearded dragons, etc). However, I'm with you, never heard of it in rats.
Actually, Brimstone111888, male rats make quite good fathers to their litters and good mates to the females (taking care of the female, feeding her, making nests, etc). The only problems I see with leaving males and females together is possible back to back litters or when keeping pet rats, unwanted litters.