Sometimes when you point out some issues straight to the others, they may not take it in the right spirit and may get mislead. That will create cohesion between us. So we should be very diplomatic and see that he will not get hurt but we are successful in conveying what we want.
If the message is to educate the other person, we should go to the level of the other person and try to explain him in a better way so that he will get the education. But if we go on delivering a lecture in your own style, the student will not digest anything and your attempt will be futile. That is why teaching requires a lot of patience and many skills. It is not an easy task.
However, if it relates to any personal suggestion to any of friends or relatives, being straightforward would not be that much effective and such communications would be counterproductive and even may lead to breaking up the personal relations. In such cases, we need to be a little bit courteous applying indirect language so that the bitterness of language is somewhat diluted but the content is understandable to the end party.
A straight forward person would never do such a bad thing. He will always speak the right thing at the right time with the right purpose. A straight forward person never creates confusion but always tries to simplify things for everyone.
Instead of being diplomatic, it is better, to be honest, and straight forward. And convey your message without any problem and ambiguity.
Knowledge is power.