In which I question the NEX APS-C Shallower DOF argument

This discussion of DOF reminds of the paradox known as 'Achilles and the Tortoise' -- it may be entirely satisfactory to a sophist, but in application it is irrelevant.
As a photographer, I'm invited to take a portrait of a CEO in his office.  I sit him at his grand desk, and have three and a half metres in which I can place the camera.  I want the imposing bookshelf behind him to be a recognizable but illegible blur.  In my kit I have three cameras and three lenses, each with an equivalent field of view when mounted on their respective cameras -- for simplicity, let's say they are 40mm on a 4/3, 57mm on a NEX and 80mm on a Full Frame.  In order to have the same DOF, the lenses would require different apertures -- f1.4 for the 4/3, f2.0 for the APS-C & f2.8 for the FF.  Or put another way, if I have a lens with a maximum aperture of f2 and the 4/3 camera, I cannot achieve the same shallow DOF without changing the nature of the shot, either reframing by moving closer to the subject (and so changing the perspective & composition), where as an inexpensive f2.8 80mm will get the job done on the FF.

As a photographer, composing a picture, it is more practical to achieve a shallow DOF with a larger imaging area as the lenses available are more likely to suit.  It is true that any given DOF can be achieved with any given lens by chasing different subject to camera distances, but that will change the nature of the picture, and is therefore not a 'benign' difference.  Or I can use a different lens, say a 30mm f2.8 on a 4/3 camera ... but then I must move closer to the subject to achieve the same framing, but the perspective will be different (and less 'pleasing' for a portrait) ...
Cheers,
GB

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