Is there wisdom which actually cannot be fully expressed except in poetry or literature or art? Or is addressing philosophical questions in such an "artistic" manner just a way of jazzing up an argument which could have stood cut-and-dried, anyway? Is there anything Homer could teach us which Plato could not?

As Nicholas suggests, it partly depends upon what you mean by 'wisdom'. Many philosophers (and others) have been attracted to the idea thatart provides a kind of experientially-based 'insight' that pureargumentation cannot supply. One possibility here is that there are properties or propositions that we (or at least, most ordinary people living fairly ordinary lives) can only become acquainted with through art. This might be because the art provides a kind of substitute experience for a reality most of us will never experience (e.g. slogging through the fog of war), or because the art provides an experience that simply does not occur in real life (e.g. the sublimity of a symphony). Another possibility is that art provides us with a perspective on, or a mode of presentation of, properties or propositions that we might already be independently acquainted with; but that this perspective or mode of presentation leads us to appreciate the familiar propositions in a more profound and intimate way. ...