I don't know why you've been downvoted, as you're absolutely correct. A defective thermostat can easily shorten the lifetime of your HVAC system. If your furnace itself is already defective, a buggy thermostat could even cause it to overheat and start a house fire.

EDIT: to those who mention the hobbyist scene, consider that such projects are not necessarily trivial, and that getting it wrong remains potentially catastrophic. I believe the parent's point still stands.