Sorry mate, but it ain't. I even checked with Mr Kwak himself, who confirmed that it wasn't. I agree that it may be useful for fitting touring gear, but if it was structural in the purest sense, it wouldn't be so easy to take it off - and I'm sure there'd be a warning plastered somewhere.
zedseven50, I must respectfully say that you, are in fact, incorrect. Regardless of what 'Mr. Kwak' told you, I can say from actual experience that the rear trellises are structural.
How do I know this? Actual personal experience.
When my V has been used solely as an around town bike, and commuter, I've had the trellises removed, and carried a top box - usually with no more than 25 lbs in it. In this manner of use I've never noticed any deflection in the subframe.
During my first long-distance road trip on the V - 12.5k miles from the Great Lakes region, around AK, and back, I had the bike loaded-down with >100 lbs of gear, to include hard side cases, the top box, and a large duffel. For this trip, the trellises were reinstalled. I apparently did not thoroughly loctite the bolts when I reinstalled the trellises, as all six bots holding the two trellises in place vibrated loose. I discovered they were loose, due to a squeeking caused by the subframe flexing both vertically and laterally under load, when the rear suspension was actuated. When I retightened the bolts, the sub frame stopped flexing. BTW, while the trellises were loose, I was also able to manually flex the subframe halding onto the hard bags. when I retightened the trellis bolts, I was unable to do this.
Is you actually look at a bare ER-6 frame and a bare Versys frame (or pictures of them), you can see that the Versys frame is missing two (if I remember correctly) reinforcing structures (welded braces, etc.) that the ER-6 frame has.