A spare engine is only doing BAD STUFF for you, ALL THE TIME, it's introducing extra freewheel, extra explosion risk, extra gear box complication and payload consumed etc etc
It has to make up for it by being useful. The only time it gets to do that is ONCE per million hours. Really obviously not worth it.
A Spare Pilot is useful more than once per 100,000 hours when the other guy has a heart attack.
He is COMPLEMENTARY, that is he actually improves the overall performance of the piloting. This is the highest yeilding safety return.
The spare pilot is useful all the time when used correctly - he is there to monitor the PF when conducting critical manoeuvres, especially IMC - not just to act as the in-flight secretary...
The spare engine is useful ALL of the time since it is not a spare but permits greater capability from the aircraft ( AUM, payload, speed etc) than a single engine helicopter. How many 6 to 8 tonne single engined, helicopters are there btw?