In The Republic, poets are banished. Why?
To understand why Plato thought that poets were so dangerous it is necessary to consider how Plato viewed the soul of man. According to Plato, the soul contains three parts: the rational part (the part that gives us the ability to calculate and reason), the appetitive part (emotions and appetites that motivate us to engage in certain activities, such as our sex drive), and finally the "spirited" part of the soul. Plato compares this part of our soul to a beast because it represents the instinctive, bestial desire for survival that emerges when we are facing severe danger. Perhaps it can be likened to the "fight or flight" response that we talk about in modern psychology.
Plato, when thinking of these three elements of the soul, believed that to be fully human it was necessary to suppress the bestial nature and appetitive nature of our soul and keep them under the control of the rational part of the soul. However, poets, according to Plato, work by arousing raw emotions from deep within us. Their talent does not lie on proving arguments, but by provoking emotional responses that cause us to act irrationally. Because of their ability to arouse the appetitive part of our soul, and give it more power than the rational part of the soul, he thought they should be banished from the perfect society.
First, Plato, concerned with finding truth, has the poets banned from his imagined republic because they are merely imitators of truth and thus distort truth. Two other problems with poetry are the following: first, poetry particularly corrupts youth by feeding it false ideas, and second, it confuses reason by appealing too much to the emotions. In other words, it clouds people's judgments and, therefore, can be used for the wrong reasons. It "feeds and waters the passions" and such deep emotions must be controlled and subdued for a state to flourish. As Socrates say:
Such then, I said, are our principles of theology—some tales are to be told, and others are not to be told to our disciples from their youth upwards, if we mean them to honor the gods and their parents, and to value friendship with one another.
Interestingly, however, Plato's ban on poetry is not absolute. He is concerned with the use value of the arts in his envisioned state, and the dialogue states that if certain poems can be demonstrated to have use-value to the state, he will consider allowing them in.
Of course, it is up to the reader to decide how much of this is meant seriously and how much is tongue in cheek.