If you have a device that you use everyday and a new version comes out that will be better in some way that's meaningful to you, I don't see a particular problem with upgrading if the improvement would be useful to you. I didn't upgrade my k3 to a k4 because I didn't really care about touch...but I would have upgraded if there had been another 50% improvement in contrast, since it is a device I use often.
If the only improvement is the glow screen, I *may* upgrade...but only if there are no other compromises and I like reading with the screen.
At some point e-readers and smartphone and the like will essentially become commodities, with only incremental changes over time. Kind of like computers or even cars are now. But while things are still in flux, marketers are going to focus on the new features, and customers are going to have to decide whether it's important enough to upgrade based on their needs and, well, their salary.