You can't always do both to the fullest truth. They often conflict. To do what you suggest, would imply my feelings perfectly align with the sympathetic view. That is not the case for a lot of humans or instances. If I am not saying exactly how I feel it is watered down.
And telling me "just do both" is enforcing your world view and that is precisely what we're talking about _not_ doing.
The "fullest truth" includes your desired outcome and knowledge that they are a human. If you just want to dump facts at them and get them to shut up, go ahead and speak unfiltered. Twitter may be an example of the outcome of that strategy.
Consider a situation where you are teaching a child. She tries her best and makes a mistake on her math homework. Saying that her attempt was terrible because an adult could do better may be the "fullest truth" in the most eye-rolling banal way possible, and discourages her from trying in the future which is ultimately unproductive.
This "fullest truth" argument fails to take into account desire and motivation, and thus is a bad model of the truth.
Its rarely the case that speaking without considering other people's feelings will be the optimal method of getting what you want, even if you are a sociopath and what you want doesn't have anything to do with other people's well beings. In fact, sociopaths are a great example: they are typically quite adept at communicating in such a way as to ingratiate themselves with others. If even a sociopath gets this, then you might want to consider the wisdom of following suit.
And telling me "just do both" is enforcing your world view and that is precisely what we're talking about _not_ doing.