And I guess I'll need to learn how this forum software works. I see I broke the quote feature, and can't seem to edit my posts after publishing. I'll figure that out eventually.
Jack
One thing to put into perspective is most all (smart) "devices" (thermostats/sensors/switches/etc) the word "smart" is really only referring to their ability to communicate with other things.
The actual "smarts" is being done with what they are communicating with (HE hub) and the automations the hub tells it to do.
My Zen (smart) thermostat pretty much the only difference between it and your dumb thermostat is that the Zen has the ability to talk to other devices with the zigbee wireless protocol. Other than that it's really identical to a dumb thermostat (besides that it looks aesthetically better)
This is also not required I could still use the Zen thermostat without even joining it to a zigbee network and would operate just as your dumb thermostat does.
There are several reasons, but the most obvious to me is what low-voltage switches and/or relays would you connect to your furnace and air-conditioner control terminals. Here is a typical picture of the terminals in a furnace (where your thermostat wires connect to) there are also terminals in your air conditioning unit. 
By the time you bought the zwave or zigbee switches/controllers for these low voltage terminals, it probably would have been less expensive just to buy a thermostat. A device designed for that exact purpose.
Furthermore, some home automation enthusiasts like to call Zwave or Zigbee thermostats "Smart-dumb thermostats". The thinking is they do no logical processing themselves but rather connect to an automation hub to do all the logical processing. This is NOT exactly true, thermostats have background logical processing that I have not seen in an automation hub. For example, my zwave thermostat will only allow for so many heating cycles per hour, it will also not allow the air-conditioner to turn back on within the first 10 or so minutes of shutting off. This logic is to protect the equipment and no setting in Hubitat can change that.
If you where to by-pass a thermostat and manage to control the HVAC through a variety of switches/relays connected to the terminals of the equipment, you better know all the safety logic that is built into thermostats (a lot more than just turn on/off heat, turn on/off cooling, turn on/off fan) and be able to replicate them in Rule Machine.
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@Stephan.J will agree I suppose
Also, thermostats are not just only relays switches. Inside a thermostat, there are rules that determine when the relay ("heat") will be triggered. And those rules are not simply on/off. There is parameters like deadbands, hysteresis and temperature curves.
Also, on HVAC system, thermostat will prevent some errors (like going from cool to heat immediately without delays). You can destroy a compressor, especially with reversing valves, if you switch too fast from one setting to another...
Ok, you can probably program this on very complicated rules (and for the temperature curves, I really doubt). But keep it simple, let the thermostat do its job and just command it through HE with the end user needs: higher, lower, heat, cool, fan auto or on.
Far easier than build complicated rules.
Black box concept !
Couldn't agree more, your post details what I meant when I stated:
The examples I provided where simple and straightforward and not meant to be exhaustive of the background logical processing built into a thermostat. I could only imagine how poorly this would be addressed by a home automation DIY'ers.
I find it interesting that people think the following parameters:
is the only logical processing done on the thermostat. You even read statement like the thermostat is just a "relay" since my hub is controlling all of these parameters. What they don't realize is the parameters the hub has control over are only the parameters the thermostat allows the hub to set. The other parameters detailed in your post are not sent to the hub and not allowed to be configured by the hub. Kind of illustrates the difference between the knowledge of an, HVAC professional and a home automation DIY'er.
Having said this, there are some people on this forum that have real talent with programming and if they understood what those other parameters where they could probably make the rules or most probably their own app to address them. It is simply way easier to buy a zwave or zigbee thermostat and have:
- Hubitat Control the Temp and Fan Setting Parameters
- Thermostat Control the HVAC Equipment Safe Operating Parameters
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A lot of solid reasoning was provided here, and we each have our certain characteristic that is important to us.
For me, I want a thermostat that can set the minimum AND maximum temperatures for both heat and cooling. This is done in the setup mode (or similar) and cannot be over ridden by the front panel (think grandchildren playing with the colored display).
John
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I would think the lock function would be more useful in that instance. 
My system is that I have Hubitat control my heating via temperature sensors and a virtual thermostat. I've put a lot of hours getting the schedule with various different modes working exactly as we want. I have a wall mounted tablet permanently displaying my heating dashboard page which gives all the user adjustments you would get on a physical stat and more. . I do have a safety check rule running every 10 minutes that checks the boiler is on or off as it should be according to the virtual stat, so if a on/off command does get missed somehow the boiler can only be in the wrong state for a short while. My heating temperature sensors are all usb powered.
It works brilliantly (if I say so myself!).
My backup plan is simple - I have left the original wireless programmable thermostat connected to the heating system and programmed with a basic 7 day timer program, but turned to standby. If Hubitat fails for any reason, I just turn the original stat back on and it takes over control. I use a Hortsmann Zwave boiler switch that has manual override on it anyway so even if both systems fail I can still switch the boiler on or off as a third backup. So far in 18 months of running it, I have never needed either backup.
"To each his/her own". Lock function would not work for me.....
- have to remember to lock and unlock when having small visitors.
- have to go into the menu
- wife would have to remember to lock and unlock when having small visitors.
- wife would have to go into the menu.
- have to remember to reset.
Just to many to do's and no fail safe.
By limiting the authority of the front panel the most you can be off is a few degrees. My heat is set on: low = 66° and High of 75°
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Ah...i was about to ask, "But they can still screw with the temp?" You have the range set extremely small.
You know there is another solution to this problem. Teach them to not touch the damn thermostat. if they're tall enough to reach the thermostat, then they're old enough to learn to leave it alone.
See... you got it. There is no need for the thermostat to have authority to exceed the expected temperature setting range.
You clearly do not have grand children. If / when you do you will understand.
John
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If you can come up with a way to impress that upon my kids that actually works (short of resorting to corporal punishment), I will gladly subsidize your home automation purchases until my kids move away from home.
To reach the thermostat the kid has to be at least 8 or 9, right? I'm not even going to comment because it'll just get me in trouble. 
According to a documentary I saw once, this isn’t a problem unique to kids:
Well, there's your problem. 
But even aside from that, there are always ways to motivate kids to do what you want. You just have to have the energy to hold fast through their whining after you take away whatever they hold most dear.
I've never met a kid yet that didn't get the picture after enough times of that. Including my own. The sooner that the accept that they are NOT in charge, and others CAN put rules on them, the better prepared for society they will be.
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