First of all like i explained before, u are only reading from 1 cylinder, whatever happens in other 3 cylinders is not accumulated for. Lets just say 1 of your injectors are slightly clogged over a period of time, and is acually spraying less fuel than the ECU actually think it is, which in turn will cause a fuel lean mixture in which will cause pre-ignition detonation, and depending on how severe the scenario is, it may cause catastrophic engine damage (this dont usually happen on new cars, it only gets worst as the engine gets older : especially when carbon builds up on the deck of the piston and cylinder head leading to higher compression ratio that builds up over time).On the other hand if you are reading from the lean cylinder, it will run much richer in other cylinders which over time will cause carbon deposit build up.

If you were to get a wideband sensor for all 4 cylinders, you will notice that all 4 cylinders will run slightly different AFR's, usually leaner on cylinder 2 and 3 on a 4 banger. Therefore by just puting a sensor on 1 of the cylinders you are actually fooling the ecu to run overly rich or overly lean instead of being at the right value. The Ecu has a Fuel adaptation function, which controls the AFR of the car, a long term fuel trim and a short term fuel trim. Whenever the AFR gets out of spec over a period of time (running overly rich or overly lean) a check engine light may come on, and this usually happend on a misplaced o2 sensor.

The reason your car is running fine could be because the ECU is always adapting the way you drive and adjusting the fuel as you go, and you may as well be in the operating range. Also i dont think u are pushing it to the limits. New cars are also equipt with knock sensors and will retard ignition timing once it senses detonation. It may save u once or twice... but it cannot save u forever. Take it to the track for an endurance race and test the longevity of the engine with sensor being placed in 1 cylinder in comparison with one reading from all 4 cylinders, which engine will last much longer? im sure u guys know where my bet is.

Im not saying that placing the o2 sensor to read from only 1 cylinder will not work, but its only better to read from all 4 cylinders.

Dont believe me on the facts? seach online and look for Original headers manufactured by ANY car make, no matter its proton, toyota, bmw or even a ferrari, it is ALWAYS after the converging point of all cylinders. Im not here to argue with anyone on the forums to say who is right or wrong, so believe what u believe and do what u think is best for your car.