ODIN vs. Nandroid backups?
IIRC, one difference is that ODIN can change the filesystem to a different "format". for example, there are ROM for the Galaxy S devices that would change the filesystem from RFS to ext4. if you did a nandroid of the device, before you flashed the ROM, then decided to go back to the "Stock ROM", your filesystem would still be ext4. This could cause issues if it is not a filesystem that the Stock ROM can deal with, or expect.

As for, "Is it better", it does make it easy for Samsung owners to go back to complete stock, but Nandroid is still a full backup of your device at that time and could, essentially, work just as well.