I get between two and four days from a Nexus 5, depending on usage. My take on it is that it's a smartphone only when I need it to be. Most of the time I'll use it for no more than email, sms and calls which I agree could be handled by a feature phone (although not as easily for sms and emails - the larger screen and keyboard come into play there).
The difference for me is that it has all the features when I need them - better browsing capability, gps and a good screen for maps, half decent camera (within the limitations of the form factor, obviously) and all the other stuff.
I have considered the alternative of a feature - or even dumb - phone for battery life and then taking a tablet when I need more, but the phone does it all for me at a convenient size. The battery lasts long enough, and I know the trade off when I use the features that make it "smart".