That duck bill tube end is about useless IMO. Darryl with tell ya that it forces your arm back so hard as to be unsustainable for use too. The round end is all you need.
My 570 didn't come with that and I'm glad. I hate to throw anything that comes with equipment away (especially for resale purposes) but I hate storing it just as much.
BTW Jason, try a 570 personally and you won't bother going with the 580.