On fixed hardware devices things are a bit easier, but consider a desktop PC with Windows. If i install a driver for my new printer, should user B also have to install that driver? Should every family member have to install a printer driver?! To make things even worse replace printer with something necessary to even run the system properly like graphics card driver. Another food for thought is windows update. I consider the keyboard localization in android to be of similar system-wide-configuration kind and shouldn't have to be redone for every user, someone else might not. (As a side note changing to a localized keyboard in android is in fact much harder than installing a driver in windows).

When does something cross the line from "system wide driver/configuration" to "user specific data/configuration"? If absolutely everything should act like a separate device you might as well dual boot or use virtual machines.

One of my scenarios for shared data is that when I'm working i want to listen to my music, but when i have a party i want to use the same device as a jukebox with the same music but not open up access to my documents. I know that i'm not alone with this problem. Trusting your friends to not dig around is another topic but with pop up notifications and active widgets on the desktop even your most trusted friends might get private emails shoved in their face even if they are trying to avoid them.