I can sort of understand why beepers where a cool sales gimmick back when the microwave was the only appliance with a micro controller, but really -- it doesn't make sense: Firstly, immediate attention is not critical when the time is up: unlike a stove or an oven, energy transfer stop the moment the magnetron is de-energized. Secondly, the microwave (at least my microwave) is not exactly silent: if you are not deaf, chances are you can easily tell when it is done.
Maybe I should apply the Joe-treatment from my old lab: whenever there was a new shipment of frequency meters for the lab (we always needed more), Joe would meticulously unbox them and stick a pointed screw-driver through all the piezo buzzers to make sure the would never make a sound.
[Edit] microtron (sic) -> magnetron