@they said
"Truth as such is an instrumental good, not an intrinsic one."
Maybe (probably). But I think that is an idea worth considering.
What if truth was an intrinsic good? Science comes to mind, as well as philosophy and sociology. What if it was intrinsically good for everyone to always tell the truth?
Would the world be a different place? A better one?
An easier way to ...[text shortened]... ow would that change day to day living? Would it possibly be an improvement from what we have today?
Not everyone is ready to hear truth (even assuming there is one and that someone knows it and is in a position to tell it -- which is not always the case).
It is easy to imagine cases, or cite real-life ones, in which telling someone the truth is undesirable. Viktor Frankel wrote a book, "Man's Search for Meaning." In it, he recounts his experiences in a Nazi concentration camp. A trained psychiatrist, he was a keen observer of humans under the most testing conditions imaginable. He noted that the people who survived the death camps were not those of robust physical constitution; given the exhausting forced labor and totally inadequate caloric intake, even the most robust physiques were reduced to walking skeletons within a matter of weeks. The ones who survived were those who believed that someone who loved them was waiting for them outside. In other words, hope of reunion with a loved one was what kept them alive.
Now, suppose you were newly arrived in Auschwitz and you met someone who had been there a while ahead of you. Suppose you came to know that the only thing keeping that man alive was the hope of seeing his daughter again (alive, of course); everyone else he cared about had already been captured and gassed. Suppose you happened to know that her hiding place had been betrayed, that she was already dead. Would you tell him ?
Further cases where telling someone a truth he is not ready to understand or cope with is highly undesirable: telling a four-yr-old her big sister got raped and murdered is such a case. Or telling grandmother, who has a weak heart and might die of the shock if she knew, that her recently deceased husband had squandered the family fortune and that she, granny, was now penniless. Better to muddle through with little white lies and wait until little sister is old enough to cope with the truth; better to keep granny warm and comfortable and feeling safe while the rest of family scrambles to find suitable lodgings for her.
In these cases, the intrinsic good is human flourishing, and truth sometimes hinders this.