I thought loose connectors, reversed aileron servos, failing servos, loose crystals, glitching radios etc were far more common reasons for failures than a poor switch?

I have only had one switch that failed on me, and that was when it hadn't been used for about 25 years. Problem was then intermittent but appeared mainly when switched on, so the preflight revealed that.

If the two batteries are only in order to avoid a switch failure there should also be an indicator for when that happens. In the scheme above, the only indication you get is a reduced flight-time as one battery has to do all the job.