Picture this common scene: A family shares an iPad, with each person struggling to maintain control of it long enough to play their favorite game, check Facebook, send email, and surf the Web. But since an iPad can have only one account, Mom can't perform such such everyday activities as checking her personal email account (unless she uses webmail), checking Facebook (without logging out and logging in with her credentials), and saving her high scores or recording the levels she has completed in a game; in addition, her bookmarks and browsing history are visible to all other users, as are any saved auto-fill information and passwords.
Most people get around this problem by not using a shared iPad for email and other personal data--but why should they have to limit their use of the device? Apple could easily fix the situation by adding multiple user accounts to iOS.
Multiples of OS X
We've been accustomed to working with multiple user accounts since the early days of Mac OS X. Logging in and out of a shared Mac is no big deal: Lots of people do it regularly, and Fast User Switching makes it a snap. Though some couples or families share a single account on a Mac, they are missing out on a number of benefits.) On an iPad--I'll focus on the tablet here because iPhones tend to be personal than shared devices--having multiple user accounts would enable you to wake it up, see a screen with several icons, tap or swipe the one for your account, enter a passcode, and access your own setup and data.
Many couples and families share their iPads; and in my role as Macworld's iTunes Guy, I receive many email messages asking how to set up an iPad for multiple users. Some people are worried about the security of their email; others have concerns about their bookmarks and browsing history; and still others simply don't want their children to have access to all of the apps on their device (very few individual apps require passwords for user access).
How it might work
A system of multiple user accounts would resolve all of these issues. Each user would be able to set up a separate home screen, and each would have personal settings established, just as happens with multiple OS X user accounts. Users would have their own email accounts, bookmarks, contacts, calendars, and so on. Also, their games would be linked to their Game Center accounts, and they'd see their personal stats, their high scores, and the levels of Angry Birds they've completed. They could all play Letterpress if they wanted to, and perhaps they'd have their own selection of apps--if the feature allowed such choices. Finally, each user could hide any of the apps installed on the device.
This new multiuser setup would entail some changes when it came to managing a shared iPad. On an iOS device that syncs with a specific computer running iTunes, the user accounts would perhaps have to be managed via a Web interface and an Apple ID. One user would still have to handle syncing. The computer would sync apps and media files, as it does now; but other data, such as contacts and bookmarks, would have to be stored on iCloud and managed via a Web browser or, perhaps, via a dedicated iCloud app designed for the purpose. iCloud (and Game Center) can already handle much of that data; the only new data would be home screen setups and app lists. To handle user accounts, third-party apps would have to hook into iCloud, too, but many of them already do this to store files or data.
To shield children from certain apps or content, the parent responsible for syncing the app would be able to choose which apps and which restrictions to apply to each child's user account; the effect would be similar to OS X's Parental Controls, and to the existing Restrictions settings in iOS.
Apple could even allow each user account to choose which media files it can sync and access. Imagine an app similar to Remote--which lets you control iTunes playback from an iOS device--that would let you choose which items to sync to your account on a shared iPad. You'd inevitably run into situations where one user would try to load all five seasons of Breaking Bad, but the iPad would already be full because another user had put three seasons of Downton Abbey on it; but to prevent space hogging, each user account could have a data-use limit--either a percentage of the device's total capacity, or a fixed amount.
Naturally, Apple would prefer that we just buy more iPads to serve each member of the household. But for people who can't afford the expense, the best way to make iPads more flexible is to allow each user to have his or her own account.