I’ve been thinking on the original of this sentence (Aristotle’s Nichomachean Ethics, book IX) for a bit. I’ll copy the relevant excerpt:

That moral virtue is a mean, then, and in what sense it is so, and that it is a mean between two vices, the one involving excess, the other deficiency, and that it is such because its character is to aim at what is intermediate in passions and in actions, has been sufficiently stated. Hence also it is no easy task to be good. For in everything it is no easy task to find the middle, e.g. to find the middle of a circle is not for every one but for him who knows; so, too, any one can get angry- that is easy- or give or spend money; but to do this to the right person, to the right extent, at the right time, with the right motive, and in the right way, that is not for every one, nor is it easy; wherefore goodness is both rare and laudable and noble.

I might not agree with his “middle ground” reasoning (I think that it’s simplistic) but I agree with his conclusion - to express anger can be good as long as you do it without misdirecting it, overdoing it, doing it when it doesn’t matter, doing it for spurious reasons, or doing it non-constructively.