That's just one use of a WB card, when the colors must be right. More common is to use the WB card as a basis to depart from along the color temperature path, without ugly green or magenta tint.
Hmmm... You raise an interesting point. When a WB card is in the image, we can do a lot with it..take out a color cast, or a 'mixed light' like fluorescent and tungsten..
However, my understanding was that the 'eyedropper' in most applications assumes that the pixel selected (or is it a group?) is either x,x,x for RGB or follows a Plank distribution for 5500K. These are not the same, since sunlight is roughly a 'black body' at 5500K and R and B are lower than G in emissivity. However, we mostly get white/grey as x,x,x yes?
Why not an 'ambient light' eyedropper? that is, select the WB card and tell it the car is illuminated by tungsten (2800K?) and will adjust accordingly.(say white as 240, 220,200)
That way green/magenta casts will be corrected but the image wont turn a warm lamp lighted image into a bluish 'daylight' look.
The bottom line is that I find in C1 esp I scroll through the profiles for the appropriate 'look' for the image. It is almost always 'warmer' than a WB gives.
Bart, since you raised the issue, do you have an idea how the WB could be used in say PS or C1 to remove color cast but preserve th 'as shot' look?