Well, only the lzo-compressed, ubinized version would be flashable. However, there are two benefits to using ubiupdatevol with the pre-ubinized version (or flash_eraseall/nandwrite if you prefer, as this would work with flashable images also), rather than the tarred file: namely, the backup would take up less space in MyDocs, which is good if you want to have several; and it gives back space on the rootfs partition (in my tests, the space before was 56MiB and afterwards 83MiB - a difference of slightly more than 10% of the total amount available).