I don't like this idea one bit. Sometimes the best offensive team doesn't hold well on D, or sometimes people can use an undergeared meta-counter to climb but it won't hold. These are good tradeoffs to have. As the person above pointed out, it increases opponent diversity in arena. If you could set a defense, then you'd end up seeing even more of the same team every day as more players switch to the optimal defense. Arena works fine as it is, and this change would have a much larger effect than its supporters realize, it just isn't necessary.