About the CAT3/L wiper scenario, I created (used) it to generate a discussion with fellow Captains here, and compare my interpretation to other's. The only place where this requirement is mentioned is the MEL, and all here said they would never refer to it in flight.
So are you saying, that if your destination airport is CAT3 weather and your left viper becomes inop, you would divert because it's a MEL item??
Would you do the same, even if the airport is covered in fog (no rain) and circumstances does not require it's use?
Remember, our job here is not only to perform safe flight, but (cost) efficient too! A diversion in the above circumstances is not efficient... hence, the one doing so is not meeting his job description.
What does your "List of Required Airborne Equipment" for CATI/II/III ops say about windshield vipers? That is a lot more relevant than the MEL (I'll bet it says both vipers must be operative "If required").