As a mechanic, how often do you come across customers that refuse proper preventative maintenance (replace one plug or coil instead of all)
For example, a customer comes in complaining his car doesn’t run right and he can smell unburnt petrol and you found out from your scan tool there is a misfire in cylinder 3. You find the problem which is a bad plug and advise the customer all spark plugs need replacement as they are past their use by date and maybe stressing the ignition coils but he is armament in just replacing that one plug.
I could only assume another plug would go bad and kill a coil which leads to more expensive repairs as unburnt fuel detonates inside the catalytic converter, leading to $1000+ in parts and labour.