<I brought the image into NX2 and then turned on and off ADL. Surprisingly, ADL took the underexposed image and made it even more underexposed. Huh?>
This was my experience as well. But I found the image with ADL Low and then turned off in NX2 was very nearly the same as my image with ADL turned off in the camera. Did you have a chance to compare ADL Off with ADL Low turned off in post?
I did a comparison of ADL off in the camera with ADL Low and turned off in post. The one with ADL Low turned off in post was very slightly different on my test shot - maybe + or - 0.1 stop. Color rendition of the background appeared exactly the same.
You're right that ADL is a bit of a black box. But I wonder if we are too quick to dismiss a function that has some merit. And you do raise a good point about introducing something that may degrade image quality.
One other thought - ADL being turned on in the camera means the image in the LCD will reflect ADL - and subsequently any blinkies or histograms will be with ADL.