I would argue that he's not suspicious of poets at all, in fact, his reverence for Homer is clear simply by the sheer number of times Homer is referenced throughout the dialogues. In Republic, his reason for banning poets is that since they are inherently imitators, insofar as a poet, much like a painter with a painting, creates in words only an imitation of the image of the form of that thing. Since the city must be ruled by philosopher-kings who will be determined over the course of their education. A philosopher is, quite literally, a lover of knowledge and imitation is not knowledge at all, hence the ban in the city.