I prefer dedicated devices that are optimized to their intended use.
Also, it would be very inconvenient for me to have one device -- I'd have to leave it in my purse or at home all of the time and never have it handy when I want it.
I do most of my ebook reading at home, and that's where I leave my reader. I don't use my phone either at home or at work. Cameras of any kind are not allowed at my work facility; if I want music at work, I need a separate device (I have an ipod nano.)