It is also an excellent concept to keep in mind if you want to get off jury duty very quickly.
I was thinking more along the line of being a defendant rather than juror, but jury nullification is a valid legal ruling to be used where an action, though illegal, is not wrong. See a lawyer if you want a better definition. If I were a juror I would be able to listen to the facts of the case and decide on their merits. Someone assaulting a person in the act of pouring blood on a member of our armed forces would likely cause me to nullify the charges. That does not mean I would see against a war protestor in all cases.