In my opinion two wear sensors are just about worthless or less than worthless because it can give you a sense of everything is fine when it is not. I have logged close to 500,000 miles on these cars and have had other pads fail big time and the one with the wear sensor is still telling me things are fine. Sometimes the other side of the rotor will fail before the wear sensor side. I live where they salt the roads so my rotors get trashed from that a lot. My first car I put over 100,000 on the original front pads but the rotors rusted out leaving only a tiny area of contact. The sensor didn't indicate anything was wrong. As well I have had sensor wires fail and tell me something is wrong when it wasn't. I usually just cut the pad wires off and twist the car wires together and drive. The best way to care for your car is to pull all the wheels off every 5-10,000 miles and inspect the brakes on all the contact points. Brake sensor wires were on the car but someone probably just cut them off to cure a bad engineering idea.
I think you're probably spot-on here. I'm not too confident in brake pad wear sensors either, especially when it's just on one wheel. I understand the idea, and it's a neat idea, but in practice, I don't know.
I actually made a huge dummy mistake. I mistakenly thought my pack of front brake pads had a wire and plug for each set - I was wrong. There were 3 similar brake pads, and 1 unique pad - the one with the sensor. I accidentally put the only sensor pad on the passenger side, thinking that both sides should have had the wiring. I got over to the driver's side, and did a huge facepalm. I had go to back and switch the pads over.
I found my drivers side with a plug and just some wires dangling. It seems someone cut off the wires from a previous brake pad like you mentioned, and tried to solder them together at one point.
I'm not sure as I've never had a brake pad indictor light come on the dash, and these wires were not completing a circuit.
Who knows...