Why lose any points for down voting answers when you are just expressing your opinion?
Ok, i'll explain .
Users doesnt want to lose their points but when theres an answer/question that i really bad they can downvote and lose some points. If you didnt lose points by downvoting. The trolls could downvote every answer on the entire site.
Simple answer: No spam downvotes
It prevents spam down-voting
Let me be clear:
Down voting should be done whenever you see a wrong answer. No exceptions.
But the -2 is so people can't abuse the crap out of it before they get banned.