"Could you guys give me some help? I'm new to D&D, and I want to know what class I should play."

"You should play an elven fighter."

"Okay. Why?"

"Because they can finesse greatswords."

"They... can? I don't see that in the rulebook."

"Well, my DM rule zeroed it. He thought it was stupid that elves had to rely on strength."

"How... how does that help me?"

"Hey, don't blame me. It's rule zero. You can't argue with rule zero."

See, here's the problem. Rule zero should never be a part of the basic assumption when someone states their problem or begins the discussion, or things become meaningless. You assume that the rules work the way they're already presented, and then, at most, you suggest things the DM might want to apply rule zero to.

We want to be able to discuss things without having to go through every possible houserule first. That's why rule zero is not assumed when talking about the RAW.