Proverbs 17:7 Excellent speech becometh not a fool: much less do lying lips a prince.
A fool cannot speak excellently because he lacks the wisdom and knowledge of God. An excellent speaker tells the truth in its entirety because he has understanding and the wisdom of God. All through these Proverbs we have seen that the truth is not in a fool and he has no understanding. We should have the discernment to know that what comes out of the fools mouth should not be heeded and we cannot depend upon what he says. The prince on the other hand needs to be above reproach because he needs to be setting a good example. It is important they speak the truth because what they say will affect the lives of a lot of people. It is kind of like us today, we need well grounded in the word of God so when we speak we know we are speaking the truth and not making a fool of ourselves.