I can think of several legitimize reasons why a family might want to do this. In my case for example my family has the following products Mac book Pro 15" a white mac-book a silver uni body and another i rarely touch, Not to mention phones as they are used individually any way. We all use each of the laptops for various tasks not the least of which is video editing and Amateur photography. I personaly would not need it for doc or image sharing as there are great tools already in existance for this. our need is redundant backup
We back up all our items to a time capsule of course but what if it fails or knock on wood we have a fire all the stuff we have Blogs, Development programs, pictures videos of all our making over the last 10 yeas would be lost forever. what we need and what most need is a safe place to have a current image of all my computers so we can recover our things . (and believe you me they are mine as i have bought them all from apple and did not make some frankin mac in my garage to cheat apple they got there money I assure you and then some)
I think it should be possible to use our cloud storage for multiple computers not for file access if we are on the road but as back up