This is a common sense policy. Likewise, it is much more prudent to be truthful at all times, than to risk credibility problems in the future. One need only look at the leader of our nation to glean how hedging the truth can harm reputation and believability. When President Clinton finally gets around to telling the nation what really happened with Monica Lewinsky, we may or not believe him. The problem is that because of past questions concerning the President's propensity for truthfulness, or lack thereof, people will always have uncertainties as to whether or not he is in fact being honest with the public.
Credibility is crucial in legal proceedings.
If a jury feels that a witness is intentionally untruthful, it is likely that witness' testimony will be given little, if any, weight. A good example of this was seen in a medical malpractice case I recently tried. The defense called a pediatric cardiologist to state, in effect, we had no case. A major issue in the case was whether a young boy died of hypertrophic cardiomyopathy, an enlarged and sick heart. On autopsy, microscopic tissue evaluation showed evidence of box car like nuclei, which are indicative of the enlarged heart.