ORIGINAL: Beavis
This is not a statement of preference for the simple, straight 2 batteries/switches to the Rx, with no battery isolating device, but there are other points to be considered that may render that simpler setup desirable. First, should one pack fail with a short, any sizeable energy transfer from the bad pack to the good one takes a long time with A123's (usually way more than a day), much more than the time we would spend flying, hence plenty of time for us to diagnose the problem when we check the batteries after the flying is over, or before the next flying session. Second, there are benefits to having 2 sets of plugs, switch contacts and connector pins at the Rx instead of just one, namely an effective 50% reduction in system impedance, which is relevant to our high drain setups. Third, the elimination of an additional item that may fail (the isolating device). So, it is understandable that there is a lot of debate on that.
Personally, I don't consider myself knowledgeable or experienced enough to make a final judgment on this. For example, it is unclear to me how important it is to regulate 2S A123's (like the PowerBox Sensor Switch does), which would be, perhaps, the only ''mandatory'' reason to use a battery isolation device - i.e. the concomitant regulation. Then again, many say that 6.6V is essentially no different from 6V for the servos and Rx we use. Dunno... What I do know is that I don't trust my planes to any batteries other than A123's.
Beavis you are assuming a lot of things here that we simply do not know and cannot accurately speculate on. The whole point of a redundant system is to deal with as many failure modes as possible. Two good, correctly speced batteries, isolated via a diode that is correctly speced for the purpose, through two switches and into two Rx ports covers pretty much every evantuallty. For example what happens if one switch should fail? Who knows? But let me give you a real life example that was fortunate to happen to a jet.
My friend used two 4 cell NiMh batteries, via two switches, into two Rx ports (one via a Y lead). He was puzzled on day when switching on that his ervos twitched once and then nothing else happened. He then switched on the second Battery. 5 seconds later he had thick black smoke issuing from his cockpit. He was fortunate that this was a jet because we had a fire extinguisher, and we quickly needed it.
Examining the disgusting mess of burnt wiring afterwards it became clear that the first switch had failed, probably a short circuit within the switch. This had flattened the first battery. The second battery had clearly seen a dead short. Why it happened in this way I can only speculate, but what is clear s that there is more than one failure mode.
In this case, had a diode been fitted to isolate the two batteries, the fire could not have occured.