Actually there is a level of difficulty to be addressed.
Files are held within an apps sandbox. If you introduce multiple users the following questions need to be addressed.
- Do all users have access to all applications (probably not if kids are involved)?
- If two users can see an app do they share the sandbox (probably undesireable ie. email, calendar and any document based app ie. pages)
- If user files are segregated does this mean each app handles multiple user sandboxes(developer impact in implementing different users)?
- If so what mechanism would you introduce for sharing files between users? ie. are you just adding another layer to the orphaned file problem we currently have with multiple versions of files across different apps.
- What if one of those users doesn't have access to the application? Does the option to share just not be shown? That'll confuse users.
- How is local storage shared between users? So if i've downloaded a massive video and used up all the remaining flash storage, does that mean no one else can use the iPad to save anything?
There are answers to each of these questions, but it's not a straightforward 'right' answer for each one. The tradeoffs have probably been considered for sometime.