Nitpick: it's not "debounce logic" (that's for handling pushbuttons, because the contact physically bounce, meaning you get multiple on/off readings within a few milliseconds), the word you're looking for is "hysteresis".
Because there's a giant surge current when you start up a motor (like that in an A/C compressor), and it's just plain bad and even annoying to have your A/C cycle between on and off rapidly, a thermostat will intentionally overshoot the set point when cooling, and then undershoot that same set point when waiting for the house to heat up before it turns the A/C back on (or vice versa for heating).
Hysteresis is absolutely necessary to avoid both this problem, and also the problem where your temperature sensor isn't perfectly accurate and the readings from it will vary a bit from reading to reading.
You are absolutely right that you need to account for hysteresis.
Debounce is also necessary for sensors that have precision problems, where, in boundary conditions, they bounce back and forth between a measurement that would cause an "on" signal and a measurement that would cause an "off" signal. But, I think we're agreeing with each other :)
For the sensors, it seems to me that a debounce routine wouldn't work, and (this is assuming the sensor is giving an analog reading, not an on-off binary reading) what you really want there is some kind of averaging routine. Usually, the way a debounce routine works is that you look for a state transition (off->on), and then wait 30ms (general rule of thumb for human-actuated switch) and look again to make sure that the state is still "on" and then you do your operation based on that. But for analog sensors, you probably want something that's more like a running average so the readings don't vary too much, and one erroneous or too-extreme reading doesn't affect things much.
I could be wrong though; this is probably highly application-dependent.
Because there's a giant surge current when you start up a motor (like that in an A/C compressor), and it's just plain bad and even annoying to have your A/C cycle between on and off rapidly, a thermostat will intentionally overshoot the set point when cooling, and then undershoot that same set point when waiting for the house to heat up before it turns the A/C back on (or vice versa for heating).
Hysteresis is absolutely necessary to avoid both this problem, and also the problem where your temperature sensor isn't perfectly accurate and the readings from it will vary a bit from reading to reading.