Using $5,000 worth of equipment and $79 in parts it takes only a couple of days to make an imitation of a device that I can buy at Home Depot for $250 or the electricity company will sell me for $150. Success!
I have three nest devices. What I want is for Google to open up the interfaces and document them so that I can program them myself. The Nest software is not up to controlling a system that has two A/C units and three furnaces with overlapping zones. And it only uses input from the sensors in the thermostats. I want the system to come on automatically when one of us starts off for home.
It is a lot harder to push those buttons when you are not in the house. The nest saves me about $1000 a year in heating and A/C costs by minimizing use. It is also in theory capable of accepting load shedding requests from National Grid. So rather than build more power stations, they cap peak load by only turning on as many A/C units as they can cope with. I don't really care exactly when my A/C comes on, what I care about is that it keeps the house acceptably cool.
What people could do to save energy is not the same as what they do. While you could turn down your thermostat every time you leave the house, I bet you don't and neither will 99% of people. In fact I don't think I want to meet the sort of person who diligently turns down the heat like that.