Ralph,
As you can see, there are a lot of divers that detest the spare air. This question gets raised often by newish divers concerned about running out of air and the repeating nature of the question and strong opinions often generate rude answers.
It is true that spare air and pony bottles are both often technical solutions for poor training, poor diving skills and sometimes unfounded fear. Not always, but fairly common and you will get the argument constantly.
The spare air concept is an air-supply so small (4-10x smaller than typical pony bottles) that is simply does not make sense as a redundant air supply. If you frame it as a small redundant air supply - it fails.
I have seen rational arguments that the spare-air can be views as a CESA assist device and I personally tend to agree that it its proper use. However, to me CESA is pretty much a last resort, a "Hail Mary" after you have failed to execute your dive in a safe manner. I would rather invest in something that has a better plan than CESA assist.
I came to the decision that with my regular dive buddies on moderate dives, I don't need redundant air. We are generally close, trust each other and know how to read our air supply. The odds of a sudden loss of air happening at the same time we somehow get separated are literally a million to one.
That of course changes when I dive solo, instant buddy or deep/cold, but in these cases the SA is not the tool as I need a lot more air than it can offer.