To be honest, while I laughed at this at first... for the angle I'm thinking is most likely a problem (people sharing a system, not people juggling accounts) this is actually a great thing. It means siblings or housemates can just get a card for themselves (perhaps the 4 GB one?) and trade the system around that way until they get their own, then use that card on their new system as if nothing happened. No weeding out saves, figuring out how to copy profiles (which would probably be simple anyway), just stick your old card in the new system.
The problem with that scenario is that I recall reading that games are made to use either
their own card or the memory card for saves, but never both. So if a game saves to its own card there is no convenient way to distinguish the two users.
Really, everything about these memory cards so far seems like a confusing mess. I can understand Sony's concerns about piracy given what happened to the PSP, but it has gotten ridiculous.