1. Because by breaching the SD card wall the user can potentially make changes to the stock B&N that will cause issues and possibly void their warranty. If the concern is to leave stock alone by using the SD card method then they probably will not want to do anything to potentially mess up emmc. This is a consideration for a user that only wants to run from SD card that they may be limited to the space seen on the SD by default by the ROM and not the extra internal space.
These are just considerations that people not familiar with bootable SD cards may want to know about.
If running ROM from a 16GB uSD card, we still have roughly 13GB left for personal storage.