Ever since its inception in Android 4.2, end-users have wondered why the multi-user function has been restricted to tablets. While switching between profiles desktop-style certainly makes the most sense on tablets, there's no technical reason why it couldn't be enabled for phones as well. Yesterday an official Android engineer took to Reddit to explain the reasoning behind the limitation.
"...it is not at all clear how it should work on a phone, specifically with respect to SMS and phone calls," writes Dan Morrill, Google Engineer and a regular on the popular /r/Android subreddit. "Suppose you have device sharing enabled and then a call comes in. Who gets it? Do you punch through to the current user? Only the owner gets it? If only the owner can answer, does it ring for the second user? Is it worse to annoy the current user with a ringing phone they can't answer, or worse for dad to miss a call from his boss because Junior was playing Angry Birds?"
Morrill cites user research as Google's primary reason for leaving multi-user off of post-4.2 phone builds. Apparently testers responded negatively to the idea of sharing phones, which are understandably more personal than tablets. (When's the last time you left the house without your trusty Android phone?) Even so, a few use cases like long-term device loans could benefit from the feature, especially now that Android 4.3 has introduced more robust controls.
Morrill says that Google may revisit the idea of multi-user on phones, if only for some very targeted use cases. "Something like a 'lockdown mode' so Junior can play Angry Birds with his own scores might make sense," he writes, though he makes it clear that the full sharing option currently seen on tablets is likely to be enabled in AOSP. In the meantime, advanced users can try mods or custom ROMs, and there's nothing stopping manufacturers from enabling the feature on their customized software.