That seems like a really small benefit at a potentially high cost, at least to me.
You can imagine that someone might create malware or otherwise hostile app that plays a loud/embarrassing sound and hijacks your volume buttons.
Having to lift a finger to turn a page seems like a really small problem in comparison to that one.
Smartphones are general purpose devices and have to make tradeoffs like this all the time. They can't just greenlight every useful function that every type of app might want. IMO if you want an e-reader, get an e-reader.
For example: Let's say I'm a private detective. It might be nice for there to be an app that records audio and video at all times without any visual indication of my phone doing so. However, having that kind of OS level permission available to apps on an app store is probably a bad idea. I'll need to go out and buy a dedicated recording device.
Sure, we can argue about where the line gets drawn. If you like Android for allowing apps to modify hardware buttons, fine. But, I would prefer a device where physical buttons perform consistent functions. A middle ground might be some kind of buttons dedicated toward custom or app functions – but, to me, why bother when the entire screen is a customizable button?