I look at it as a personalized phone directory to the school, broken down by class. We're discussing this in our PTA, and we're thinking you could still be listed by address, but not have your phone # listed if you don't want.
I can imagine it'd be VERY helpful to those who are new to a district, or not as involved as say someone like me. Homeowrk questions etcetera? Just look up a kid in the class and give a call. Birthday invivtations that can't be handed out on school property? Especially if some people aren't listed in the regular phone book, you now have a place to send these invitations if you don't know the family's address.