I agree with you. After having just seen a performance of MacBeth, this is fresh in my mind. The witches are not prophets, but are more like those crafty carnival fortune-tellers who are good at reading faces and body language. These witches also are adept at analyzing situations, especially political climate and personal ambitions. They know that MacBeth would like more power, but does not really believe it will happen. So they put the idea in his head that he will be King. The more he thinks about it, the more he likes the idea and talks it over with his wife. she takes the ball and runs with it, and together they plot the murder of King Duncan. Without the witches' nudge, would this ever have taken place?