Knowing when you're home can also be done quite well without any Internet access - just see if your mobile device recently associated to the wifi.
These things can trivially be built without the privacy concerns. The business model might be a bit harder if you can't collect the data, though...
The thing I really liked about the Nest is that I just turned the dial to where I felt comfortable and it just took care of everything else for me without me needing to do anything. As long as you're brave enough to put screws in a wall and hook up a handful of wires to connectors, it took about 15 minutes of me getting it out of the box to it being installed and configured. I can't say the same about any other programmable thermostat I ever had. Seriously it wasn't an awful lot more complicated than the mercury switch thermostats I've installed.
The handful of Honeywell programmable thermostats I've installed had a disastrously bad user experience. In fact, I installed the Ecobee in my current house because I got so pissed off going through the programming cycle of the Honeywell I gave up before I got to Wednesday :'D
Also, it knows when you're not home, so it can then just shut down everything until temperatures would cause other problems such as frozen pipes.
Additionally, you just dial it to where you're comfortable and if you get too hot or cold, adjust it again. It learns what temperature you like it at at certain times of day, it takes a week or two to really get to know you, but once it does, it just keeps the temperature comfortable and you rarely have to touch it or even look at it after that without you needing to go through a complex programming process like you do with most other thermostats.
That's pretty much what a smart thermostat gets you.
Whether or not you consider that useful or a good use case for your needs is really down to your own evaluation.
I like that my thermostat is smart enough to be able to regulate the temperature in my house without needing to kick on the AC or heating, which reduces my power consumption somewhat and is somewhat better for the environment, but is also better for my wallet.