A typical scenario:
The human shoulder.
Typical armature, one bone for the shoulder, two for the upper arm, two for the arm. The shoulder vertices must be influenced by the shoulder bone and the first upper arm bone.
I want to paint the upper arm bone weights. Then copy them to the shoulder bone, and invert them. So, if a vertex in the upper arm bone has a weight of 0.8, the same vertex must have a weight of 0.2 in the shoulder bone.
This operation is typically done by hand, and it is easy to do so when the topology helps (all quads, low poly and straight edges). The problem is that this time the topology is complex and very high poly, it looks very good but I'm having a hard time weighting the shoulder.