It would seem that the above proverb conflicts with the requirement that we are held to in a court of law where we are instructed to tell "the truth, the whole truth, and nothing but the truth", but what is missing from this picture is that fact that a court of law is all about getting the full picture of what has occurred and the knowledge of this so of, which thus negates those that would like to spin things in a way that would favor one side regarding a particular issue at the expense of the other. In other words, our courts of law need the whole truth, so as to consider fairly the information so provided and thereby to rule correctly in that court of law. On the other hand, we in our day-to-day activities are not held to the restrictions of a court of law, but are held to the standards of civility and sensibility which signifies that there are those times when discretion is necessary because it isn't so much that the other person cannot handle the whole truth, though sometimes that is the case, for it really comes down to us being emotionally intelligent in how we communicate, which does not mean that we are thus permitted to lie in certain circumstances, but reflects that we owe it to the other, to be prudent in how we communicate with them, in the sure knowledge that not everything truthful must be divulged. This signifies that we should take care in what we say as our civil obligation to one another.
So too, when we consider that not all truths are to be told, it needs to be understood that in life, we, in a lot of respects, earn our right to the whole truth, and that to provide the whole truth for those that are not prepared for it can be damaging to that person, because they are not ready to receive that truth, or do not know how to handle such truth being revealed to them at that time. Indeed, as parents, there isn't any point in certain cases to telling the whole truth to a child, especially when all it will do is confuse the child or disturb the child's mind, which again doesn't mean that we lie to the child, but rather that there are those times when our experience, our wisdom, and our discretion overrule simply divulging the whole truth when it really isn't appropriate to do so.
In life, it needs to be recognized that to achieve great things necessitates a good foundation to build upon, and that the edifice that we thus create needs time to be developed, so that overwhelming somebody with information that serves no useful purpose at that time can even be detrimental to them and should not be given out. The timing of anything of real import is critical, so not all truths need to be told because the time is not right, because, for example, of the unpreparedness of the other, or the simplicity of their mind, or their lack of maturity, or their lack of openness, which is why we who are wise are careful with imparting the most profound truths, because that is our obligation to the whole truth, and why we should always be careful and considerate about it.