Coming from Softimage to Maya after Fable 3 we spent periods of our development time writing tools we were used to having. One such tool was a dedicated Weight Editor to fill the missing pieces of functionality within the component editor.
This was written in PySide and OpenMaya. As its written in python it certainly hits a performance limit with large numbers of vertices. However, working with focused selections is no problem at all.
Architecturally the code is split into two primary modules, one to handle all the Ui and callbacks and another which holds a class which wraps around a skin cluster and exposes data in a way that makes it quick and easy to access and manipulate skin information from the Ui class.
In terms of actual features, these are what the weight editor offers:
Copy/paste between multiple cells
Ability to lock/unlock influences
Slide value data by percentage, absolute or addative values
Apply stepped values addatively, negatively or absolute
Save and Load skin presets
Auto/Manual update based on selection