Plato insisted that the emotional part of human beings can be useful only when the rational part maintains complete control. For seekers on the path of sophrosune (the path of moderation) Aristotle's catharsis made as much sense as plunging the philosopher back down into the cave periodically so that the light of knowledge would dazzle him more thoroughly. Even today, for those who desire a reasoned, ordered existence, logos represents rationality, and it is to be found in an education that privileges the logic of philosophical precepts.
It is this education that Plato revered. It existed in contradistinction with the educational style of the Sophists, the new, controversial educators in 5th-4th century B.C. Athens. Sophists inherited the "pathetic" tradition of poesy, rhapsody, and oratory. Impressive words and emotive force countermand wisdom, which can only be found in an education that stresses logic and rationality. Wisdom therefore comes through philosophical logos, not Sophistic pathos.
The person easily swayed by emotional appeals cannot be relied upon to rule a country. That which Plato called the 'ancient quarrel between poetry and philosophy,' is at least partially a response to the tradition of hostility to exploitation of the pathos stimulated by strong emotional response. Beginning with Plato then, pathos becomes any state induced by strong emotion, and strong emotion is inimical to reason and rationality. The developing use of pathos in Ancient Athens, however, exists in contrast to its earliest meaning of a true victimization caused by the gods, a catastrophe largely or entirely undeserved by the victim.
However, Socrates does not accept the change to the use of the word pathos. For Socrates, the gods are always good, and stories showing gods acting otherwise cannot be allowed, hence the rejection of rhapsodes and poets from the perfect city-state. The cause of one's trouble is always self-inflicted; sufferers must be seen to deserve and need their punishment.
This response to pathos politically situates Plato's argument against emotion, because the real debate for Socrates (and for Plato, of course) was the debate between philosophers and Sophists. It also helps explain Plato's distaste for those he considers the purveyors of a poisoned vision of reality, those who blame the gods for terrible occurrences--the tellers of the 'great mythoi,' the poets, and the 'lesser mythoi,' such as mothers, wet-nurses, and the pedagogoi (tutor) the young man listened to prior to going out into the world.
Ultimately, Platonic logos is an intellectual defense against oratory as enchantment--the orator's ability to bewitch the audience with persuasive, but, Plato feared, empty rhetoric. The power of the Platonic word lies in its ability to create a reasoned, dialectical argument with its only goal the search for truth. And this is our Western philosophical tradition, the underpinning to our societal prejudice against emotion. Along with that prejudice comes every other prejudice we have against writers and the irrational realm creative types inhabit.