Why are poets banished in Plato's The Republic?

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.

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.

Why were poets expelled from Plato's Republic?

In understanding why Socrates expels the poets from the Republic, I think some background is needed.  The argument here is that the philosopher- king is the only one capable of understanding the forms, or the essence of truth.  It is they alone who comprehend truth and have the responsibility of telling all of us what that truth is.  Socrates sees a danger in having others who perceive to know what truth is assume the role of truth teller.  Socrates argues that poets and poetry tells us images of the truth.  The reason for this is because poets and poetry are more concerned with the lyrical nature of the story and see the truth as nothing more than setting for their tales.  It should be noted that he is aiming his critique at less than competent story tellers, such as novices or others who seek to make a living off of telling stories.  I am not sure Socrates would be able to level this criticism at an Sophocles, Aeschylus, or Homer.  Regardless, due to the fact that the standard storyteller is more concerned with their story and its lyricism or "prettiness" as opposed to the serious and focused pursuit of truth, they pose a threat to the entire kingdom because people will become confused with the truth, as seen through the philosopher's eyes, and the images of the truth, as depicted through the poets'.  It is because of this that Socrates expels the poets.  If we want to take this to a modern application, there are modern artists who would like to confuse us with their pursuit of art as opposed to a more serious and truth- ladened journey towards the form of art.  For example, Beethoven's art should not be considered even close to the same category as Britney Spears'.  The former was driven by a pursuit to expose musical notions of truth, while the latter is driven by something else that might confuse both issues.  If we applied the Socratic method in this setting, he would argue that Spears should be expelled because of the tendency to confuse her presentation of the musical truth as something that could even be close to presented along side Beethoven's. (To any and all fans of Brit, sorry.  Just an example.)

What is the consequence of Plato's banishment of poets?

Plato wants to banish poets from his ideal republic because he thinks they would distract people from the pursuit of the truth. Poetry makes people emotional, thinks Plato, undermining the full exercise of the soul's rational faculty. And for the arch-rationalist Plato, this is the most important faculty of all. Ultimate truth can only be grasped by the intellect, not the emotions, and poetry leads us astray in this regard.

Plato's understanding of poetry, as with all art, is essentially mimetic. That is to say that he regards the arts as imitating nature. An essential element of Plato's philosophy is that the world around us—the so-called phenomenal world, the world of other humans, animals, plants, and objects—is merely a copy of what is ultimately real: the world of Ideas or Forms.

As Plato believes that poems imitate nature, then they represent nothing more than a copy of a copy. In other words, they are at two removes from what is ultimately real. Poets themselves are therefore not committed to the truth, and so represent a real danger to the stability of Plato's ideal republic. Once the poets have been banished, Plato thinks that society will be more moral, more stable, and more consistently devoted to the pursuit of rational wisdom.