I agree that anger makes us irrational. We may say or do things in anger that we would never dream of any other time. Anger tends to lead us to lash out at the ones closest to us, which are often the ones we love.
On the other hand, post one has an interesting point about the positive side of anger. Anger can lead us to create major changes in the world around us. Not that long ago people were angry at the injust treatment of African Americans. This anger lead people like Martin Luther King and Rosa Parks to do things they normally wouldn't have done. These things seemed irrational at the time; however, we now look on these actions as great catalysts of a much needed social change.
One character, a law enforcement officer, in the old TV show, Hill Street Blues, would frequently say that he was "becoming irrationally angry." Somehow this phrase seems a bit redundant as most of the time people who become very angry are, indeed, irrational when they experience such volatile emotion. Therefore, the disadvantage to becoming angry is that the fiery emotion and adrenaline in one's system often excludes rational thought. Such a situation is usually dangerous, of course.
You may wish to research studies of the brain that explore the areas touched when people become angry, the neurological changes that take place, etc.
When we get angry, we tend to lose sight of our long term goals and interests and act only in ways that are "helpful" to us in the short term. For example, we say things or do things in order to hurt the person we are angry at or to get revenge on them. These actions "help" us in the short term in that they make us feel good. But in the long term, they hurt our relationships with other people.
If you are writing an essay on this topic, perhaps you could try to think of incidents in your life where your anger has caused you to act in ways that were harmful to you in the long term.
This question looks as if it will be moved into the discussion posting section of this group as you will gain a wider range of responses, but as I am first, let me contribute my ideas!
If you had control over your essay title, a far more interesting essay title would be to focus on the advantages of anger, as we normally associate anger with negative outcomes and as a negative emotion, however, I will go with what you have specified.
I think the biggest disadvantage about anger is the way that when we are angry, we lose control of our bodies and minds to a certain extent and do things that we regret later. Words are said, actions are carried out, that are done in the 'heat of the moment' but are not really meant and would never have been done were it not for anger. Thus anger can be said to help focus our negative emotions and feelings in an unhelpful way, causing damage and hurt to those around us.