You wrote: "I watched an entire 3D movie just holding the unit"
Same here ! In fact, when I got the Cinera, I found it so light that I never felt a need to use the arm. I think it holds well in the hands and it's not tiring at all. I find that much more comfortable than the damn straps on VR viewers that literally push the viewer into one's face.
The only reason I can think of to use the arm or the FPV unit is if one is a gamer and holding a game-pad or of course if one is using it in conjunction with a drone. And I guess the arm can be used for a movie when one just wants to lie back without having to hold the unit. But personally, I found it very comfortable just to hold it.