I would think it's that SMS and telephone features don't map well to multiple users. Would you share them across accounts or block them for all but one user? It's just weird.
Anyway, we've been there already, and it worked for the most part. We used to have one phone in the kitchen, one in the living room, one in my sister's room and we would all be fighting to not have to be the one to pick it up whenever it rang.
But it worked, and it wasn't exactly "weird" :P