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.