The reason why it might behoove someone to wait until someone has "cooled down" is because, regardless of whether or not you think it shouldn't affect someone's decision-making capacity, emotion has a strong influence on judgment. Not everyone is you, and it's fairly limiting to have one blanket expectation that you use for everyone. Under stress/ emotional duress people will respond differently, therefore making the best way to respond to any situation being to respond to people on an individual level, rather than making unrealistic expectations of them that they're never going to meet - because, once again, they're not you. It would be spiffy if people could respond in a straightforward, rational manner no matter they're mood, but, unfortunately, people are human. Consequently, their mood is going to compel to respond in ways that could be completely baffling and irrational, and ways that they might regret later (also things that they wouldn't have said otherwise).
I would imagine that humans have this response for a reason - emotions driving actions as, for example, a means to survive. Unfortunately, at the same time, it can also get people into trouble.
You could come up to someone who's pissed the fuck off, get in their face, and tell them what you think they've done wrong, but do you rationally expect them to respond in a logical, straightforward manner? They're not going to be reasonable, they're not going to hear what you're saying, but this is a natural consequence of the strong influence that emotions have on one's judgment.
I do, however, strongly agree that using one's emotions as justification for causing poor behavior is unacceptable, in most cases, but that doesn't change the fact that it can influence behavior.