Re: 2011 Sienna Roof Rack: Can I Haul This...?
Since the roof rails handle all of the load on the crossbars, they must be able to handle it without the crossbars; on the other hand, I don't remember what the tops of the rails are like, and whether they would be suitable to directly apply load (the crossbars insert into slots on the inboard side of the rails). I can't imagine there being good tie-down points without the crossbars - there certainly are not on the second generation Sienna. A bigger concern would be that the furniture is unlikely to sit well directly on the curved rails, even if the spacing between them happens to suit the furniture.
An alternative to buying Sienna crossbars would be to get generic aftermarket towers/feet and crossbars/load bars of whatever model fits the 2011 Sienna, from Thule (foot model 450 or 450R) or Yakima. They will work on other vehicles, perhaps one that you actually have or will get, in case the Sienna is not purchased; they would also have better resale appeal than Sienna-specific parts. Warning: these are not cheap (hundreds of dollars for a set).
Personally, I would rather tow a small trailer, but I realize that's not an option with a rented Sienna (which would not have a hitch or trailer wiring).