This is a question coming up in our family too.
Son is going to Australia in August 2013 for 1 year. I just assumed we'd be able to book a return ticket for him when prices looked good and then pay the change fee to push the return flight further into the future after he takes the outbound. As I understand it, the amount of available cash he needs to qualify for his visa is different if he has a return ticket bought so there is financial incentive to have a return ticket in hand.
I knew this isn't possible with a reward ticket but didn't realize it wouldn't be possible with a revenue ticket as well.