The goal of a White balance tool is to get an impression of ... ambient reflections, that will determine how the subject color reflections will look....
...Also remember that the scene illumination mostly comes from illumination sources that are behind the camera and to it's sides. Measuring (averaging color reflection) of what's in front of the camera only makes sense if it is of a subject with known spectrally neutral reflection (of that illumination behind the camera), or the same color as the illumination.
Well, I guess that depends on the nature of the scene and the illumination. As you and others have pointed out, putting a grey card in the scene may not always be convenient. I would think this is especially true when there is no definable object. Or to put it another way, when the whole scene is the object (ie no tomato to use your example), such as in a landscape. The scene may have light sources in front and behind the camera and from the sides, for example street lights at night, which is my particular case. Pointing the ExpoDisk (or any of the other similar plethora of products) at the source would be a challenge. But then again, I would think it would be a challenge for a grey card as well. Or the scene may not have illumination coming from a particular direction, but may be generally amorphous such as on an overcast day.
I would think that if the goal is to get an `approximate` calibration of white balance for later post production that pointing the ExpoDisk at the scene would give reasonable, average results. The disk will be lit up from the reflected light from the various objects in the scene, much as a grey card may be lit up by reflections from the many objects.
Hey, either way I guess I will give it a shot. Its not that expensive. Or maybe I`ll just look for a suitable bottle of salad dressing
Cheers and thanks
Robert