Very well stated, FLS! That's the main thing that has prevented me from triple monitoring. You expressed my vague thoughts in clear English.
Already about 10 (ten) years ago Kanttori and I envisioned the future of gaming monitors. We fantasized about half- or even full-spherical screens, where you could just look around as in a real plane. Our debate went on about if it were better to have a hemisphere in front or above oneself. Ultra widescreen as in triple monitoring is one thing, but just as you said, in aviation games it lacks the vertical aspect.