JCMs are morphs - which move vertices along a straight line between the base position of the vertex and the morphed position - that are controlled by the value of another parameter, such as a bend. Weight-mapping is an alternative way of deciding how strongly a vertex responds to a bend (or other parameter), like the current falloff angles and spheres (and capsules and extra zones in Poser 8+). They are totally different things. There are two desirable aspects to weight-mapping - first, that it allows any weight to be applied to a vertex rather than being restricted by the falloff shapes which should be a boon with more complex items; and second, as long as it's a single-skin system, that you are no longer restricted by the group boundaries (to choose a common example, it would be possible to have a webbed hand and the finger bends would affect the skin between fingers; the current limits on grouping - mesh may touch only its parent and its children - and deformation - a bone may partially affect only itself and its parent - prevent that.