But it doesn't cause any issues:
1. rear sensor isn't used for engine control, so it can't do anything. so yes - I will warranty something that can't happen.
Not true. Rear O2 sensor data is used to assist in STFT to help control the catalytic converter temperature. The reason a front sensor is switching is to relay a confirmation of what the ECM is doing to maintain a temperature control without overheating the CAT. Switching from lean to rich is a cooling effect. In the old days, air pumps cooled the CAT. When the pump failed, melted CAT. The cars would run with a bad CAT and you wouldn't know it, yes, unless it clogged. With new programming of the ECM and new AF and O2 sensors, data is much quicker, reaction timing is faster, the CAT is cooled more efficient with less parts and efficiency of the engine and CAT performance is seen in many ways, as well as a longer CAT life.
The rear O2 sensor data will also tell you if the engine is running too rich or lean when a known good CAT exist. Knowing how the sensor operates helps to find the real problem.
2. rear sensor can be completely removed, unplugged and car will run indefinitely with no issues (see #1)
Also, not true. It will run with the rear sensor unplugged. It will also run with the AF unplugged. It will run with the IACV unplugged (with some creativity). It will run with the EVAP system disconnected (Although you may want to be careful removing the gas cap). It will run with the ECT unplugged. It will run with the fans unplugged.
But in order for the ECM to operate the engine in a manner that makes it perform and put out the power it was designed for, all these need to be plugged in, operational and in the correct position. Leave that rear O2 unplugged long enough and the programming changes to a lesser output to try to save the CAT. Pull it out of the air stream and the ECM will add fuel to cool the CAT because it will think its getting hot. Given enough time, carbon will begin to clog the combing. All ECMs are programmed to save the CAT while putting out the performance. My VDC is. I want the sensor in the exhaust so if something happens that I don't see from the gauges and extra gauges in the car the ECM will see it and say "Hey, something's up. Find it before it causes damage.". I can't drive around while watching data all the time and I can't record it in 20 minute or hour plus intervals for later review.
3. older Subaru's never had rear O2 sensors and it caused no adverse catalytic converter failures or engine problems because of lurking engine issues people never knew without the P0420 code. actually it was the opposite - catalytic converter failure was almost unheard of for 1980's and early 90's Subaru's, before there was a code for it.
Again, CATs would fail and you wouldn't know it until it clogged up enough. And running and running proper are two different things.
I try to explain things simply. Every sensor on that car has a purpose and some do more than you believe them to. That rear O2 is apparently one of them. We aren't talking about a 1987 GL. It was basic. Nothing has been basic since 1995 Japan markets. These newer cars are programmed for power output increases with lower amounts of engine volume. It is accomplished through computer programming and ignition/fuel mapping that works much faster than your pen and paper trying to calculate fuel density, air density, air temperature, barometric pressure, water/alcohol content ratios, weight of the car, gear and gear ratios, drag, tire size, and throttle opening effectiveness. Its not a carburetor. Remove the data or alter the data the ECM receives and the programming changes. You won't notice it right off. Just like you don't notice a difference between using Wal Mart gas over a major oil company blend. But its there. And it kills. Slow like cancer. Sometimes quick.
Give people a choice. Sure. Here's the choice: Protect the investment that you rely on daily, or, take your chances and roll the dice. I choose the first. Anyone choosing dice, try going without your or any car for 3 days, normal work days, everyone in the family, then think about it again.