People as dissimilar as the Buddha and Mark Twain have observed that nursing anger -- that is, unnecessarily prolonging it for whatever reason -- is unwise since doing so tends to harm you. Do you agree or disagree? If so, in what ways can nursing anger harm you? If not, why not?
Yes, I definitely agree and disagree.
Nursing anger is definitely dangerous and harmful. Alas, it is in human nature to often need to harm ourselves, out of ignorance of better ways if for no other reason.
Anger isn't a good thing, but it is entirely common to be in situations where the realistic alternatives are even worse.
The trick, the actual important decision, is IMO to nurture situations which will make the need to express anger less likely. Which is to say, we should all strive to learn to anticipate situations which are likely to anger us and create the circunstances which will make them less likely to manifest.
There are two main reasons why we don't always, out of the top of my head.
The first, and perhaps most significant, is plain ignorance. It is not rare for people to just fail to make the connection between their circunstances and habits and their anger, or to realize which options they might have to break it. Some environments are in fact designed to purposefully raise anger, often for religious or military purposes. Anger has a social use in that it makes people predictable, easy to control. Not a good thing, but an appealing one for some.
The second is some combination of fear and self-importance. Or better yet, just call it lack of faith. For one to decide to avoid being angered, there must be some degree of faith that it will be worth the trouble, that people will cooperate enough for the effort not to be wasted.
So, in a nutshell: anger just is. It is not in itself particularly bad. But living a life where anger is rarely avoided is symptomatic of a danger of becoming disfunctional, abusive and parasitical, or otherwise unhinged.