Because rhetoric normally is concerned with arguing about opinions, it is the case the positions taken on some issues may be equally valid, even though the people who hold them think their position is obviously the right one. And even in regard to issues in which one side is generally perceived to be superior to the other, it is important to know both sides of the argument. Therefore, it was a common practice to ask students to speak or write arguments for both sides of a controversy.
A good way to do that is to pretend that you have been asked to be an advocate for someone. After reading a text or hearing a position, create the best argument you can in defense of it. Think of yourself as a hired lawyer, or spokesperson. What are the best arguments you can make in support of your client's position? Then pretend like you have been hired to argue against the same position. What are the best arguments you can make against the position?
By putting yourself as fully into each side as possible, you begin to see the internal logic of each position. This insight is important for several reasons. First, it may help you to be more understanding of your opponents' position (they're not always the fools we think they are when we haven't explored their position carefully). Second, it may make it possible for you to find some area of common ground between the two positions that will produce cooperation rather than arguing to "win." Third, even if you think the opponents' view are wrong and must be defeated, you at least know what arguments they are likely to use, and you can figure out how to disarm those arguments ahead of time.
But what do you do if you feel strongly about your position, and you feel guilty even listening to the other views, let alone taking them into yourself so far that you are able to write them as though you believed them? When you first start doing this kind of exercise, such qualms are common, but it is much better to take the "poison" (if that's how you think of the other view) on your own time and under your own control than to find it being used against you in the heat of battle when you don't know how to deal with it. So, on the one hand, you can overcome your qualms by assuring yourself that you are preparing yourself to make an even better argument than if you didn't write on both sides.
Even more important, though, from an ethical position, is the value of exploring the other view so that you can be fair. Is it always the most ethical thing to win your argument? What if your opponents have some truth in their position? Isn't it more important, if possible, to find ways to cooperate than to push toward polarized positions? Consider places where long-term animosities are destroying countries. This is not to say that one should compromise his or her beliefs, but one reason people argue their side without listening to the other is that they are insecure in their beliefs. A good way to overcome that fear is to explore the thing you're afraid of; perhaps you will change your mind, but perhaps, you will come away with a stronger sense of security in your own position.