Version:           2.0.0 (using KDE 4.6.4) 
OS:                Linux

Other RAW apps use two curves which has some advantages in general and could have some for digikam's usability and ease of entry to RAW manipulation.

The base curve fits the camera in general and the luminosity curve would allow to alter the luminosity further and individually for the current image, starting from the base curve without having to touch the base curve. the luminosity curve would be the fine tuning if you want to put it that way and the base curve the starting point.

Another advantage would be that one could ease the luminosity's curve handling like it is done in darktable. This would not affect those that find darktable's curve adjustment too limiting, since those users can continue to use the base curve and manipulate it as they always did.

Applying darktable's way of adjusting helps to fine tune for those that do not want to fiddle with the points on the current curves. These points have issues. For example one often greps the wrong one, e.g. when working in the lower-left corner of the curve and having several points in there. Clicking and dragging a point close to the black point's point will very often drag the black point.

Darktable offers "sliders" at the bottom of the curve which the user can use to manipulate the curve. It's best to try it to understand why it is a lot easier than digikam's way of handling the points on the curve. 

http://darktable.sourceforge.net/documentation.shtml

Reproducible: Didn't try