The #1 reason I'm switching to a nook is eReader compatibility. I've been using a Sony for two years but I still have dozens of eReader-formatted books from the 8 years I spent reading that format. If B&N don't offer epub replacements for our purchases, it still shouldn't be too hard for them to maintain eReader compatibility on their devices forever. The DRM scheme will likely stay the same and the peanut formatting is dead simple to decode, it's essentially a very basic HTML-type markup language.
My understanding is that B&N (and Sony) will be transitioning all their books for sale to ePub format, so hopefully we can re-download all our purchases in the new format. It seems silly to duplicate titles in their system, rather than overwrite the old versions entirely.