defringe: mask vs. global

In a mask we can use the Defringe control and it works well.  Sometimes I create a mask over an area using a brush or gradient solely to use the Defringe tool.  Why is Defringe in a mask so easy to use, effective, and without the bad side effects of the complex Defringe in the global tools?

The global Defringe tool rarely works well and often it doesn't work at all.  It is common to get a message with the eyedropper that the area is too small or it can't detect anything despite it being clearly visible.  And when it does work you have to play around with the 4 sliders and almost never get a satisfactory result.  And on top of that the real killer is that if there is anything else in the photo that has a similar color (clothing, car, etc.) then parts of that can also be changed to monochrome.

I sometimes even create a Linear Gradient over the whole image if I have problems in many places in order to use the mask Defringe which works so well.  So much better results than using the global Defringe.

Why can't the simple, easy to use, effective Defringe in a mask not be also available in the global area?