While I agree that this sounds like all the drawbacks of an ereader with none of the bonuses of a phone, I think compartmentalisation is a pretty big factor. If the back of your phone is for reading, some people will be less likely to get distracted while their phone is flipped over - even though they’re literally holding the likely source of their distraction.
Edit: I feel stupid, but it occurs to me that you don’t have to have it attached to the back of your phone. I’m actually considering getting one now…