There are people who by default want to tell you everything they can. If there is something they're required to keep back, they find this difficult. For such a person, a special reason is needed not to say something, a reason such as confidentiality, not boring the listener, decency, etc. These kinds of people are annoying—they are apt to tell you more than you want to hear, to spill secrets, to gossip and to offend. That's the kind of person I am. On the other hand, there are people whose default mode is silence, who require a special reason to disclose something. These people are great if you want to run a conspiracy, but their value is found in many other circumstance—they won't bore you by telling you what you don't want hear, their speech is more apt to be modest, and they will only offend if they choose to. On the other hand, it is easier for them to be deceptive.
In a sense the first attitude is the right one. The argument for this is simple. Knowledge of the truth is intrinsically good. It is intrinsically good to bestow a good on another. Hence, it is intrinsically beneficent to tell someone what one knows. But if an action is intrinsically beneficent, then the presumption is in favor of doing it. One doesn't need any additional reason to disclose what one knows other than the fact that the listener will gain a piece of knowledge she lacks.
Of course the attitude of those who like to disclose has its dangers, namely that there are many defeaters for disclosure. And the danger of being a person of the sort who has a presumption to disclose is that one will neglect the defeaters. These defeaters are almost all, and perhaps all, grounded in human fallenness. When one needs to keep quiet about something, it's generally because something has gone wrong or is likely to go wrong. But in a fallen world, things go wrong quite often. Actions are likely to have unintended consequences that could have been foreseen with more thought. In a fallen world one might need to have a presumption in favor of measuring twice and cutting once. Moreover, in a fallen world, we need to be careful when speaking, because we may simply be mistaken, and we have a special responsibility when saying something to ensure we're not speaking falsely.
If so, then there is something right about both attitudes. The discloser is right that there is a presumption to disclose knowledge. But the non-discloser is aware that in our fallen world some of what we think is knowledge isn't, and the world is full of defeaters for intrinsically good actions. The discloser may get right the axiological structure, the rule to tell what you know is indeed generally right, but the non-discloser gets right the fact that defeaters are very common—the presumption for disclosing is quite weak.