I think you misunderstand me. The image we boot from on the micro sd card is a clean install (after all it's just there incase anything goes wrong). So you have a clean install of maemo on your micro SD card - you backup your N900 rootfs using this clean image to access the mtd from here and also restore the mtd from here. But yes the whole point would be to be able to boot from micro sd card and also backup and restore both opt and rootfs.