Isocrates appears to be very practical when he is comparing what he does to the work of the sophists. The sophists, or so I've gathered, saw oratory as something that left room for pleasure. Isocrates' perspective focuses on the usefulness of the message. To argue for the sake of arguing about some trivial absurdity isn't worth his time.
He also reveals just how long some of the debates we find in composition studies have been raging. He speaks to the question of whether a good orator is good because of training or innate ability. His answer is that talent lies in nature. Someone who can fashion a message well has a natural ability. At the same time, a person who doesn't have much ability can become better through practice--and may even surpass someone who has natural ability and doesn't hone it. In the end, though, his faith is in natural ability, and he believes that a good orator can be developed, not trained from scratch. I believe that there are still traces of this debate within modern composition studies texts.