Someone might agree to that because they believe that yes honesty is the best policy because for one, you are not lying to them. You're trusting them with the honset truth. Another reason someone might agree with the statement is that lying is something you will have to keep lying about. If you tell the truth, you'll be able to forget about it after a bit and not keep up an act. The last reason someone might agree is that it's slightly less stressful than lying and you're getting it out and over with.
But Someone might disagree with the statement because The truth can sometimes be harsh and something you don't want to hear. The truth can be used to hurt someone emotionally. The truth can also be a form of black mail.
(hope this helped :)
