To put it another way, a properly used pony is in a binary state. It either has air for me underwater, or it does not. If it has air, I'm going to use it. If it does not, I'm going to the surface. A pre-dive check will tell me if I have air, and I'm keeping my pony off during the dive, so if I need it, I will have air. I don't need an spg to tell me that.
Scuba gear is incredibly reliable, but those tiny o-rings in your spg are, in my experience, the most failure prone bit of gear the average diver carries. You said you think they aren't a significant failure point, but they are a failure point.