I think the idea is that being curt is a mild evil that is tolerable if the information contained has a high probability of being valuable. But if the person being curt (or even rude) has no particular bearing/merit in the listener/reader's mind, it just comes off as pushy or presumptuous, sometimes to the point where you are tempted to ignore what's being said altogether.

It's not ideal, but I think the basic version is: I trust you enough to put up with your "brutal" honesty, vs. who are you and why are you being a dick?