I do. I have a command line nest app that I use to control my temperature.
edit: yes: https://www.ecobee.com/home/developer/api/examples/index.sht...
The root of my question though is this: Is most API use a hobby thing, or a professional thing businesses are build around?
My assumption here is that the financial impact is trivial for them. They aren't Valve ending Half-Life modding for example. If that's not a clear analogy, Valve's enabling of modding for Half-Life enabled Counter Strike, which basically kept Valve alive so they could become what they are now.
There are lots of mostly invisible companies selling high-end automation that don't need to advertise so you'll never have heard of them. It's not just hobbyists that build this stuff.
I have eight Nest thermostats across two homes, and a pile of Nest cameras. My "hobbyist" use of the APIs collects and aggregates some of the data together since the Nest UI (web or mobile) is lousy at best. Given that Google Home / Google Assistant do not work* with my G Suite account, I'm not at all confident about continuing to use Nest equipment after the grand changeover to Google Home Assistant Whatever in August.
* By "do not work" I mean: You log in to a Google Home device with a G Suite account and it (the device) cannot be shared with any other accounts, and it cannot access any of your data like your calendar. Oddly Alexa has no difficulties accessing my G Suite calendar.