Jim and Rune, I agree that yours are the lightest and simplest systems with fairly reliable backup. But there are some weaknesses:
- What happens if the BEC fails in such a way that the output is shorted? Likely the backup tries to power the system but depending on the short circuit resistance, may deplete too fast.
- What if the BEC puts the full input voltage through when it fails? In this case the backup battery may go up in smoke and flames as well.

Of course these failure scenarios must be extremely rare and in most cases the backup works well enough to land the plane.

I have a similar system, switching BEC that outputs 8 V and a 2S 500 mAh backup battery but additionally I use a dual channel switch that has voltage regulators set to 7,1 V inside. This totally eliminates problems with the first scenario I listed and limits damage in most cases should the BEC put a too high voltage out. The weakness is that I have to charge the backup battery now and then, typically after 9 or so flights. I could fine tune the switch regulators so that the battery channel would output a bit lower voltage which would eliminate the need to charge almost totally. Now the load is shared almost 50/50 between the two sources.