If the A/C is not running, the clutch on the compressor spins freely, so there is no loss of economy [and I DO note you are a diesel owner].
Granted, the car is not exactly a hot rod, with or without the A/C, but a subsequent buyer may want that feature, which may not necessarily add value, but will make the car more marketable.
If it helps you to answer my question or just ease your curiosity, I'm not concerned about subsequent buyers, the car is not for sale.
I live in Seattle, when it's nice I roll down the windows and enjoy. The front of the compressor pully rattles with belt attached or not and with AC on or off. That bugs me. Dragging around any unecessary weight and working around it for maintenance and repair also bugs me.
Additionally I would carefully plug the hoses so no debris or moisture can intrude into the other A/C components and also secure them so that no rattles, noise or damage can occur. That way, should you decide to re-install the A/C you will not be looking at replacing any extra parts.
I suspected that it also had something to do with the extra weight. I know that "diesel doods" march to the beat of a different drummer. I once sold a diesel wagon to a dyed-in-the-wool diesel guy, and his first comment was along the lines of removing the roof rack as doing so might get him an additional 1/2 MPG...
I'm in the middle of other under hood issues and thought about this, not really a MPG issue, I drive about 3 miles to the train and hop on it to work.
It's basically my urban tractor.
As far as the lower ratio, it was also built in the era of the nationwide 55mph limit so it doesn't care for too much above 60, but 1st gear is a stump puller.