Q: Why UBI does not use OOB area of NAND flashes?

Because many flashes (e.g., NOR) do not have OOB and UBI was designed to be generic. Also, modern MLC NAND flashes use whole OOB area for the ECC checksum, so there is no room for application data.

But of course, things could be optimized for SLC NAND flashes if UBI used the space available in the OOB area. This is not implemented, but one could probably do this.

출처 : http://www.linux-mtd.infradead.org/faq/ubi.html