For the non-geeky audience, that is to say: not us!, the question is really, "How many e-reader do I need?". Other than the frontlight, there haven't really been any innovations that make a new device an urgent purchase or, really, a reasonable one.
I'd not be surprised if the average "usage-span" of an e-ink reader was several times that of tablets or smartphones. It's one of those devices that you buy and then use for years without any pressure (or even incentive) to upgrade to new models frequently.
A tablet will always benefit from more CPU power, higher resolution, better battery life (or a fresh battery in case of the models where you can't replace it), more storage space, etc, and apps that you want but can't run are a steady reminder that it's time to upgrade.