It really is that simple.
When it comes to BIG engine performance it really isn't. Redundancy is good, certainly, but its only part of the reason.
We used to get a very "electrical" smell throughout the avionics workshops whenever the magneto test rig was in use - which, with a resident Shackleton squadron was most of the time. There was also a separate side shop in the engine workshops where they cleaned, adjusted and tested the Griffon spark plugs. As stated in earlier posts, magnetos and spark plugs are not the most reliable devices and they require constant and detailed preventive maintenance -
and don't forget the ignition harnesses!
Did you hear about the Shackleton captain who arrived back at base with all four engines running? He had to shut one down as he didn't know how to do a four engine approach. 
The old ones are best... Ta-taa!