On long backpacking trips my smartphone is all I have for multiple things, including books, and I agree --- works great. In fact, there are some aspects that make it better. It's light, as you point out, very easy to hold up with just one hand. I also like the fact that I can program the volume keys to represent page forward, page back. I find that much more easy and natural to turn the page by keeping a finger or thumb over the appropriate volume key to page forward effortlessly and sort of unconsciously. With a kindle touch it takes a poke or gesture that requires more energy and doesn't always work.

Biggest advantage, though: the phone is along with me in various scenarios in which I might not have brought my tablet or my wife's kindle, or the tablet/kindle might not be available.