I have some experience in this situation with a SOB 5'ver that I traded in for my Big Horn. The dealer told me the same thing.....the weight of the water would cause the awning piston to retract and the water would run off the sides. That was NOT the case during a heavy rain. The water collected in the middle of the awning and I was not able to retract the awning due to the increased weight. I was fortunate to be present for this test run so I was able to manually move one side to get the water flowing. I'm convinced if I was not there I would have experienced some damage to the awning or RV. It may have dumped the water on its own if I waited, but I did not want to chance it. Add a little wind to the scenario and I think you're in for a problem.
I contacted Dometic/AE and they sent a kit to the dealer for installation. While the kit was free, the dealer socked me with $109 for installation, The installation required the simple removal of 2 pivot pins to relaceme the original arm with an adjustable arm.
Fortunately the BH 3580 that I bought was manufactured in October 2009 and the awning had adjustable arms on both sides. I would suggest installing at least one arm to prevent any potential damage. The install is very simple, but I don't know if Dometic/Heartland will supply it w/o having installation by a dealer.