-E-ink can be stared at for long periods and in bright light; staring at a tablet screen for long periods gives many (including me) eye strain.
-Tablets usually include many other apps and internet access, which is a huge distraction when you just want to focus on reading a book. When I'm reading, I prefer to just read.
-Good tablets are much more expensive, so people might hesitate to drag them all over the place like they would a paperback.
The technology is already pretty cheap to produce, and I imagine big players like Amazon, BN, Kobo, etc. will eventually just sell them for a token few bucks and make up the remainder in walled-garden sales.
In the end their usefulness depends on what kind of reader you are. If you read for just 30 minutes a day, then a tablet will probably be fine for you. If you spend hours with a book, a dedicated reader is nice just for the e-ink.
I haven't heard anything new lately on how e-ink technology is continuing to advance, so I'm wondering if they've hit some kind of snag where they just aren't advancing fast enough to be very relevant outside of the ebook niche.
I have a second last generation kindle, the refresh time is fine I think for the primary purpose.
Tablets are one place where we would love to see e-ink solutions (with color and a bit better refresh). Or consider e-ink wallpaper. The possibilities are many, but the tech needs to break out of its rut before its too late.