The continued flex of the cover back could result in premature wear, cracking (leather) of the cover and is yet another reason that I choose to remove the K3 when using it. The cover is quite adequate for protection, but never seemed to me to be suitable or even intended for use while reading. Like any book backing, the number of times it can be flexed is finite.
If you have children who could knock the Kindle on the floor, or if you use it on a train or subway or bus, then using the cover is probably a good idea to protect edges and the screen.