Hmmm, you could always use the fear angle. Explain that the more you're playing WoW the more you are NOT out smoking crack with phillipino prostitutes ...

In all seriousness though, I'd figure out the pros of 5 boxing first. What it's going to accomplish for you, IE the fact MBing 5 means never having to LFG. Make it a solid argument for the jump to 5. And then present your parents with the facts.