I see your point, but that is really only the case in a single user environment (so why have a "user" anyway). But, in my case, the default user for public areas is "Guest". Guest is all public media, and is used since we entertain. The default user in our bedroom is the wife. The default user on my laptop orbiter is me... etc, etc. If someone where to switch to a user (Don for instance) then Don's private media should be available as well. Don could of course choose to use a pin if he really needed his media private.
In my situation, all family members have their private media so that the 100's of movies are easier to browse. I use a pin, and the rest of the family doesn't care if someone looks at their private media; I just want to organize the media so that I am not looking at 300 cartoons from my kids media collection, and my kids are not looking at my 100's of my adult movies ;-) And I know it can be done through tagging, but seriously that does not solve the issue. I may want to look at dramas, just not the 300 drama's in my wife's collection.
I can see a big need for this, especially once LinuxMCE is more mainstream. Especially for organizations, like a daycare or school. Users could actually be by use... such as teachers, age groups, classrooms, purpose, etc.
Also, if it were an option, then someone can choose whether or not to view private media when the user is selected.