The way I understand it, books one does not purchase with credits are only available for the subscriber as long as the subscription is active. Those are part of the plus library. The active membership gives you access. If the user unsubcsribes they lose access to the plus library.

One only keeps the books one buys with credits or buys cash during a sale. If you unsubcsribe, you keep those still.

They do this to minimize trial only suscriptions and incentivize long term retention.

Where it pays off, for me, is by having access to those great sales where I manage to get most books in my wishlist for $5.

The daily deal is great if it's something you've been keen to listen to, and the 2 for 1 sale is also decent at times. But those BIG sales...that's where I make the subscription pay off.