I think perhaps you misunderstand what I'm arguing for. It's not about being able to see the merits of the opposing viewpoint (although that is a very beneficial effect of it, especially when the issue in question is highly polarizing). I brought it up because it was useful to me to see the issues in my own perspectives, but that wasn't why my teacher insisted that our assignments be written that way, or why I continue to do so.

Ethics and philosophy don't have right answers, so picking an answer and going with it is the only option, and the chosen answer is rarely significantly less valid than the opposing point. The only thing that matters is whether your conclusion follows from your premise, and the best way to practice that skill is to argue things you don't actually agree with. It's very easy to say "Because x, y and z just make sense" when it's a point you agree with - for example, it's very easy for a pro-choice activist to answer the question "Why does life not begin at conception" with "Because that concept is fundamentally ridiculous." That doesn't make it a good argument, but it's very difficult to avoid doing anything else when it makes so much sense to you that it's just an intuitive truth. Clearly, though, these aren't universal truths to everyone. Learning to argue from the opposing side is useful for two reasons:

First, it is an effective way to learn to prevent yourself from arguing yourself into a corner. There are plenty of arguments I've seen on Reddit where one party has said something that seems mostly reasonable in the context given, and the other has taken that point to its logical conclusion (which is a slightly insane one) but, because both parties were more invested in feeling right than being right, the other person tried to argue that their original point wasn't crazy, and it devolves very quickly into meaningless arguments of semantics. Learning to arguing from an opposing viewpoint teaches you moderation. You aren't nearly as likely to take a view you don't agree with to a crazy extreme by accident, and when you're accustomed to doing that, you become naturally more capable of recognizing those pitfalls in the arguments you do agree with.

Second, being familiar with the main points of an opposing group is useful when discussing things with them. Like it or not, there are certain (particularly ethical) conversations that are more important to our generation than others. Talking about the perspectives of racists isn't especially useful because few people alive are left to defend "separate but equal" with any real seriousness. My familiarity with the stances of pro-life activists, on the other hand, actually matters.