I too would like to see this on some quizzes. It would be nifty if there were a checkbox you could select that would cause the results page to show % of players guessing each decoy answer, in addition to (or maybe instead of) the main results.
In some quizzes you can achieve a similar effect by turning your question around, ie, instead of asking "Can you click all the xxx that are yyy?" rewrite it to be "Can you click all the xxx that are NOT yyy?"