in sidemount, I'll just stay on that bottle until I can feel a buoyancy difference, switch to the other side, and when they feel unbalanced in the other direction I'll know that the "dead" side has more gas than whatever is in the "alive" side.
Not to be argumentative, but this is a okay measure if you are not stressed. Frequently, problems in cave diving multiply and snowball, and stress becomes a huge factor, and things like this aren't noticed. I have seen some crazy things in incidents and accidents, and when you detail the situation and the amount of stress, then it is understood.
If the xmitter battery fails early in the dive, and you have an spg there is no need to abort the dive
This is the assumption the transmitter fails, but the receivers are not infallible. I love my Shearwater and think they make highly reliable equipment and quality equipment. But I have mine go "tits up" a couple times in the cave, by my error with a battery that was dying, or some other unknown reason. I had a back up computer of another manufacturer, so no problem, which follows your two primary light analogy.
I am not anti-technology,but diving in the overhead, be it recreational or exploratory deserves equipment that has been well tested by a large population, and highly reliable. The analog spg has a highly reliable history in the overhead, and don't recall an accident due to a failed spg. But, wireless spgs need conservative dive profiles and X number of hours of bottom time to show a reliable history.