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.)
In Plato's Republic, Book10, Socrates argues that the poets are to be banished from the ideal state, the Republican City, for all poetry is based on 'Imitation'. No thing is true; every thing is an appearance or illusion of the idea of the thing. A thing is once away from reality/truth; what a poet creates is an 'imitation' or appearance of an appearance, and thus twice away from reality/truth. Socrates refers to 3 kinds of a bed--the bed made by the divine; the bed made by the carpenter[once away from reality]; the bed made by the painter[twice away from reality]. Thus the poets are liars, and therefore they should be expelled from the City of ideal governance.