To form your own opinion, you need to understand both sides as much as possible. Often times, like in some school debates, people are picked at random so the side they are put on may not be the side they agree with. This skill would come in handy for some careers such as a defense attorney who has to argue in defense of his client even if they know he is guilty. One Movie comes to mind. Inherit the Wind, where the attorney who defended the teacher who was teaching evolution was a devout Christian.
On the other hand, if your a sheeple that is told what opinion to have. By all means, ignore what the other side has to say. Just don't bother getting in a discussion with anyone that opposes it.
On a side note: I was once a Christian and my family still is. Tho I am now an atheist and tend to speak on their side, often times I refute atheists who either disrespect people of faith or if they base their opinions on idiocy.
Arguing against yourself would be a complete disadvantage. You should not have to do that because you would not argue truly and would not have enough information to counter your own information. If someone is challenged to a debate for something that they do not want to argue, they should not accept.