Speaking of avoiding points of failure, an old 220D has no glow plug relay and no glow plug fuse, so it is already ahead (?) of some of the more modern cars in avoiding points of failure.

The reason I like the series GPs is that I know right away if one of my GPs has failed. My understanding is that depending on which GP fails with the pencil plugs, your GP light may or may not indicate a failure. I never have to wonder. (keep in mind I live in a mild part of CA, so a complete failure of the GP system is not life threatening)

My 220D has the 5th GP filament on the dash behind the steering wheel. It is a pretty neat way to know how bright the other GPs are glowing. If I went to series plugs, I would need to bypass that indicator. It would be sad to me to lose one of the very unique features of my car.

Also loop GPs typically do not typically need reaming. Also, I have never heard of a loop gp getting stuck in the head.

But at the end of the day, my preference for loop GPs is primarily that I like the quirky aspects of the car.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).