Somehow, I doubt that with Buell's small staff they had time to be generating useless patents.
I don't think you'd necessarily be adding any weight to the swing arm. None of that baffle material has to be particularly heavy gauge and it would serve to stiffen the swing arm, possibly allowing material thickness (and weight) to be reduced elsewhere. Getting the exhaust from the engine to the swing arm would no doubt take some careful engineering. IIRC, the patent shows alternative rotary joint. It's not that hard to do.
You do it because it's better mass centralization, and some folks don't like to see a muffler/resonator hanging off the side of the bike. I have NO idea that EBR will actually do it; I'm just saying here's one way to get around having a muffler hanging off the side of the bike.