I stick Mrs Azrael's "Mario Kart" into my 3DS XL and all her profile and saves and unlocks are there.

How do I play it as me?

Anyone know?

I don't have a 3DS so I can't specifically help you with that, but on the DS games save to the cartridge itself. The system memory only stores data/saves for digital content. Simply put you have to manage the save files within the game itself.

Feel free to correct me if I'm wrong, I don't want to mislead.

I remember from the old days of having a DS that the games saved to the cartridge.

Usually you can navigate to a file management section but they just don't seem to have one...

That or I simply haven't found it yet.

Aha! Hold down XYAB at the title screen to erase all data.

I suppose if they let you have multiple profiles this would discourage households from purchasing seperate copies.