Exactly, which is why we should all obey the POH and not rely on advice given elsewhere if it contradicts.
Individual magneto switches present a different situation to a rotary, combined switch. In an aircraft fitted with individual mag switches, I would certainly agree that a dead cut check is unnecessary.
Personally I would prefer individual mag switches as I see them as a safer way of doing things.
I know of one case where one of our RAF Bulldog aircraft developed a badly rough running engine and was unable to maintain height. My colleague who was on an instructional sortie then discovered the rotary mag switch was the problem and it was intermittently grounding out the mags. Going to "L" and "R" didn't cure the fault. He was able to recover the aircraft by holding the switch in a certain position just away from the "BOTH" setting while his student flew the aircraft to the overhead where they cut their losses and flew a forced landing pattern to the grass.