What is the effectiveness of the figure of speech used it "Enthusiasm for cause is most commendable but without personal discipline it is as useless as having a good donkey and a good cart but no harness"?

This quote is spoken by Mr. M during the debate that begins the play. He instructs both Thami and Isabel to calm down, since they cannot get their points across when they act "without personal discipline." The simile sounds traditional, almost biblical, showing this perhaps conservative side of Mr. M's character: especially as he is seen by Thami.