Might be that I don't trust a frost protection function and I don't particularly like the thought of a gas water heater running at night while I'm sound asleep. What if it don't cut in for some strange reason like you forget to set it, the pump battery fails, ignition batteries fails, run out of gas etc etc.
To be honest it only takes seconds to drain my gas water heater so convince me would should I bother spending 200 squid on another controller with a frost function that may or may not work?