Okay, I think that is a legit thought. My bad for my tunnel vision. I sometimes forget about sport diving in favor of thinking about things in tech diving terms. On a tech dive, if one computer died (e.g. your Vision), the dive would be over anyway, so the redundant SPG would serve no purpose. But, you make a valid point. On a sport (aka NDL aka recreational*) dive, if you have 2 computers and one dies, you don't automatically have to thumb the dive.
That said, even on a sport dive, I still would not take a redundant SPG in the water. I wouldn't do it with a primary pressure gauge being a physical SPG (and I've never seen anyone else diving 2 physical SPGs). And I definitely wouldn't bother with a physical SPG as an installed backup when I was using AI for my primary cylinder pressure gauge. But, this does seem like a somewhat legit consideration in favor of using hoseless AI over a hose-connected console. With hoseless, I can have 2 computers both reading the same transmitter, so if 1 computer dies, I still have my other computer AND can still see my cylinder pressure. With a hose-connected AI computer, if that computer dies, there is no other means to see the cylinder pressure (other than, as you say, another device, like an SPG, connected).
Also, one would hope that on a hose-connected AI console computer failures would be rare enough that the risk would not really merit installing a backup SPG just for the benefit of possibly saving one dive from having to be thumbed early. In my mind, you'd be trading off the insurance against ending a dive early because the computer died against the chance that the SPG has an issue and you have to end a dive early because of that. Sort of you're damned if you do and damned if you don't.
*I'm trying to avoid use of the term "recreational" because, in fact, tech dives are also recreational dives. But, there's no denying most people use the term "recreational dive" to mean a sport dive, aka a "no stop" or "no decompression" dive.
