"...having a vested interest is THE requirement for judging over a case."

I actually didn't say that it was necessary to juries. What I did say, or meant to say, was that interest improved the ability of juries to judge because it caused their attention to be sharp and focused, and their interest aligns with actual justice.

Anyway, I don't think juries are magical. They certainly reflect their societies, which includes the bad with the good.