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The first set would be used as a "contributes to discussion - vs - does not contribute/flame" indicator, while the second set would be used for "I agree/support this view - vs - I disagree/don't support it".
The usage of the voting at the moment seems a bit unsymmetrical, the up button is usually used to show support of the argument, while the down button is used, correctly the most times, to indicate a comment that is inflammatory/troll or doesn't add anything to the discussion.