Failsafe.
OBDII is not a failsafe. It can be a warning, but it is oblivious to many problems. No codes does not mean a properly functioning vehicle. Relying on OBDII to tell you if you're rich/lean is like waiting for your debit card to be declined to determine how much money you have in the bank.
And yes, crap in = crap out. If your O2 sensor is giving crap readings, the car can be running lean but the ECU won't pick up on it until there is some other kind of catastrophic failure. If OBDII is telling me my car is running fine, but it's not, is it wrong? YES.
Adding a wide band gauge adds another layer of protection. Can the wide band fail? Yes... but more than likely you'll recognize that the readings aren't making sense. You'll also have OBDII as a backup. The odds of both sensors failing simultaneously are so small they're insignificant.
A light isn't instantly triggered when fuel is requested but not received. How many times must fuel be requested, how often, and for what duration before a code is thrown? I mean hell, I installed my MAF sensor backwards one time, and the car went crazy until I figured out what had happened... but no codes were thrown. After that, I had ZERO faith OBDII would catch a problem. Try it for yourself, and then tell me OBDII will save you from all your problems.
Btw, every time the car is started, it goes through a light test. The check engine light worked. The O2 sensors are fine. The pump wasn't fine. OBDII didn't catch it. End of story.