I don't know why this is a problem. Remember that you would have to restore a NAND from the device itself. So if the 3DS were to be stolen, somebody would have to restore YOUR NAND backup using a hardmod. For that to happen, the 3DS thief would have to know where I live, know where I keep my NAND backup, break in to my house and steal it. I think at that point I have bigger problems than my 3DS