This is a great question and there are two ways to answer this. First, we need to know something of Plato's view of humanity. He believed that all people has three parts: the mind, heart, and the lower regions. The mind was the seat of reason, the heart was the place for emotions, and the lower regions was the home to base desires and appetites. Based on this three-part break down, Plato believed that good and noble people ought to have their minds control everything. With that said, Plato also believed that poetry could corrupt a person by having their appetites control them rather than their minds.
Second, Plato had a view of imitation. In Greek it is called mimetic. He believed that poetry makes us imitate things that are not noble. For instance, often times in Greek poetry, we read about the worst things such as when wife kills husband (Clytemnestra kills Agamemnon), or when when a father sacrifices his daughter to the gods (Agamemnon sacrifices Iphigenia).
Finally, we could also say that Plato believed that the poets knew very little; they were not philosophers. In a word, he had a very low view of them.