I've read this passage a few times, and if I understand Epictetus' point correctly, then I think he is saying something along the lines of this: rhetorical argumentation has its use, insofar as it is applied to leading the person to a proper education (in philosophy) and as long as the person keeps the focus on its proper use, which is the development of a good character. However, as the person is still not fully educated, there is danger in learning and using rhetorical argumentation because it may lead to him becoming "conceited and presumptuous (v. 8).
Epictetus warns more explicitly,
For how on earth can one persuade a young man who excels in these studies that he should not become an appendage to them, but rather make them an appendage to himself? Won’t he trample all these appeals underfoot, and walk about among us full of pride and puffed up with conceit, never being willing to allow that anyone should try to remind him of his shortcomings, and of where he has gone astray?
We should learn rhetoric and argumentation in the pursuit of living well and living in agreement with Nature. If we become good at these skills, it should be because we are good philosophers - we love wisdom and virtue and wish to explain and argue the philosophy well. But, some may go astray by becoming good at rhetoric and argumentation and take pride in these skills, rather than taking pride in living the good life.
Keep your eye on the mark: wisdom, excellence of character, virtue. Don't be distracted by the skills that augment the embodiment of your philosophy.
Epictetus concludes the point:
Plato was strong and handsome, is it necessary that I too, sitting here, should toil to become strong or handsome, as though that were essential to philosophy just because a certain philosopher happened to be strong and handsome as well as being a philosopher? Don’t you want to understand and distinguish what qualities people require in order to become philosophers, and what other qualities may be present in them accidentally? Come now, if I could be counted as a philosopher, would you need to become lame like me?
Now, do I want to deny you these capacities? Heaven forbid! No more than I would wish to deny you the capacity to see. All the same, if you ask me what the human good is, I can offer you no other reply than to say that it lies in a certain quality of choice.