Actually, I think a PS only needs to appeal to a very specific audience - the adcomm. Everyone else doesn't matter. It needs to be a reflection of you, why you would add diversity to the school, and your reasons for pursuing a career in law.
My dad was a lot of help because he is pretty scientific in his approach to writing (opposite of me, I tend to appeal to emotions). I think its really helpful to find someone who is stylistically very different from you because they can point out things that you would never even see. A PS needs to appeal to the widest possible audience.