No, it has such a big impact on how they act in the house, if they didn't hear the crowd or have guests/new housemates who tell them how they are perceived then they wouldn't have any idea of who was popular and who wasn't and would probably be more honest in nominations and friendships.
In the past new housemates were punished for sharing any information from outside the house.
I don't mind the letters from home as a nice one off, but apart from that it should just be what goes on in the house.