I have seen issues where heating systems either failed to work, or got "stuck on" due to a number of reasons. In one case it was the failure of an electronic thermostat to call for heat, possibly due to a lightning strike. Another time, an elderly person turned the heat up as high as possible to warm up the house fast, and forgot to turn it back. While I am at this point only discussing heating issues, I realize similar things could happen on the cooling side...
Having the heating way too hot, or shutting down when nobody is around could either waste a lot of fuel or lead to pipes freezing, etc. Vacation or weekend homes, houses for sale, etc. might fit in this scenario. (I have heard stories of potential buyers adjusting the heat in vacant houses to see if the heating works, then never setting it back to where it should be.)
I like the idea of the setback electronic thermostats, and have used them in my home for over 25 years. Some of the earlier ones did fail, but the newer ones seem to be more reliable. My current set-back thermostat is over 15 years old, and has never given me any trouble, but I still worry about what might happen if the thermostat were to fail when I am away for a few days.
I am curious if anyone has added additional thermostats to provide some redundancy. First, a standard heating thermostat could be installed in series with the first and set to a "maximum" house temperature, say 75 degrees. This could be wired to interupt both the fan and burner, or only the burner. I would probably use something like the old reliable Honeywell bimetalic thermostat in this situation, and that the thermostat itself could be installed perhaps in the basement near the furnace where it is unlikely to be fiddled with. In the event the electronic thermostat fails in the "On" mode, or someone tampers with the settings, the house temperature would be maintained at an appropriate level.
A second mechanical thermostat could be wired in parallel across the electronic thermostat, perhaps in the basement again, and set for a minimum temperature. This might be 45 degrees or something. In the event of failure or tampering of the main thermostat, this thermostat would maintain a minimum temperature, keeping pipes from freezing.
With the addition of the series and parallel thermostats, the minimum and maximum accepted temperatures could be preset. (This might be appropriate in rental units where the landlord pays the fuel bills.)