"I believe myself able to speak about Homer better than any man; and that neither Metrodorus of Lampsacus, nor Stesimbrotus of Thasos, nor Glaucon, nor any one else who ever was, had as good ideas about Homer as I have, or as many."
Plato's main purpose in Ion is to differentiate between gift of true knowledge and gift of shallow speech.
True knowledge is not limited to one artist but must expand beyond mere information about one person's work to an entire brand of study. For example someone who truly appreciate the written word will notice every new book that comes out, will analyze works of many writers and studies literature in depth. This happens because he is naturally attracted to the power of the written word and hence his soul awakens every time someone mentions something exceptional they have read or heard. But the same was not true for Ion whose knowledge was strictly limited to Homer and even on Homer, he was not an authority because while he could give dramatic rendition of Homer's epics, he couldn't possibly analyze the work in depth and had no ability to discuss the real meaning of Homer's philosophies.
Socrates tried to explain to the rhapsode that he was not an expert on Homer or on poetry for that matter, but was only inspired by Homer's work. Some one who is truly blessed with knowledge is not someone who is just inspired by one artist but is interested in the actual power of the force that drove many artists in a given field. However rhapsode in this case was a mere interpreter who had been inspired by one artist as Socrates explained: "The gift which you possess of speaking excellently about Homer is not an art, but, as I was just saying, an inspiration; there is a divinity moving you, like that contained in the stone which Euripides calls a magnet…In like manner the Muse first of all inspires men herself; and from these inspired persons a chain of other persons is suspended, who take the inspiration. For all good poets, epic as well as lyric, compose their beautiful poems not by art, but because they are inspired and possessed."
Thus the whole text has a contentious nature. Plato was frustrated with lack of knowledge about rhapsodes and hence chose to explain to Athenians that a man of knowledge doesn't have to be someone who possesses the gift of flowery speech. Thu using Socrates as his mouthpiece, Plato tells Ion that a real artist is the one who is not moved by the teachings of his teachers to the extent that he would try to adopt their profession. What this meant was that no body can become a good general or a good anything by only learning about them. This argument took place when Ion said he had read so much about generals that he thinks he could become a good general. But Plato wanted people to understand that everyone is suited for a different job and one's main aim is life to recognize that job and do it wholeheartedly.
In this way, we can find a similar theme in Ion and Republic. In both books, Plato argues against assigning jobs to people who are unnaturally suited to them. Though the topic is discussed in a different light and under different circumstances, the fact remains that in both works, Plato shows his displeasure with people's inability to recognize the tasks they are more suited for and then taking over jobs they cannot do well. In Ion, he chastises those self-proclaimed masters of knowledge who think they can read about something or someone and become that person, and in Republic he charges against a society where people fail to recognize their strengths and hence often undertake tasks they are not naturally suited for.
The Republic Online Version: Retrieved from http://classics.mit.edu/Plato/republic.mb.txt
Ion by Plato Online Version Retrieved from http://classics.mit.edu/Plato/ion.html