Up until now Android didn't have a concept of multiple users. Think back to Windows 98 that had a single My Documents directory and proper multiuser support only arriving with Windows 2000 and its Documents and Settings folder. I could draw so many parallels here.
The key point remains that unlike the migration path between Windows 98 to 2000 and having an internal hard drive vs an easily removable SD card, there is a certain expectation Android users have - seamless OTA updates and backwards compatibility. The /sdcard partition and paths on it have been mistreated so much in the past few years (just look at your card and see how many apps don't store data according to guidelines). If Google were to resolve the path issue and repartition the card upon upgrade to something like /sdcard/user1, /sdcard/user2, etc, it would suddenly become incompatible with other devices. Plus there's still no proper permission support.
Robert Mahon: Another thought about this sd-card stuff, might it have caused issues with the multi-user side of things in 4.2? Most apps aren't following guidelines on where to save their data, so it gets sprayed willynilly on the SD-Card. Now there's more than one person using the system, and possibly overwriting that data/corruption, I can see why Nexus devices, to stop this being a problem before it starts (or at least reducing it to less apps) is not using the sd-cards.
Stewart Gateley: With each user having their own apps and data, SD card makes even more sense. Keep the profiles local while storing large media files on ext storage. Either external files can be shared with all users, or set file permissions. No need to partition space out for each user or something. For a multi-user HD media consumption device, the Nexus 10 is seriously lacking storage. But I thought we were talking about the 4 here.
Dianne Hackborn:
+Robert Mahon This is certainly an issue, we aren't supporting multi-user with the old school FAT partition for external storage (whether that is on an SD card like the original G1 or a separate internal partition like the Nexus S).
+Stewart Gateley SD cards must be formatted with FAT (to inter-operate with desktop computers), which doesn't support file permissions. Not to mention that all you are going to end up with is a mess if you try to set file system permissions (based on uids) on an SD card and then move that to another device.