Well the 65mm or 2.5" is the average human distance of the i/o and for this a Stereo set up to match human vision needs to be set up at such i/o distance, and this is called ORTHOSTEREOSCOPY, but this is not the standard, even so you can shoot tone of stuff with it, there is no way to match a 3D rig with both i/o and convergence control, and more so with A image splitter set up, as they just simply give infinite adjustment capability.
Both i/o distances and convergence are used to create different perceived image distance, size, volume, and obviously depth, were we control not only the size and or 3D effect but we control were the subject is placed inside or outside the screen.
But also the most important things to understand, or at list one of the most important, when shooting S3D, is that the i/o distance controls not only the effect of the given captured image, and create Giantism or Dwarfism effects, but it also dictates the minimum distance in which the camera can be placed, and or must be placed, while convergence focusing dictates were your you are placing your subject in great control.
For the above reasons, a 3D rig must offer extensive and absolutely precise control of the camera movements, usually one fixed and the second one fit to make all control settings.
There is so much to Good S3D that I just have never more in love with the art then since I started studying 3D MovieMaking, and promised my self that I will shoot all or almost all my project in S3D, I just simply love it.