Left to Right:

1. V4 Default.

2. V4 17K LOD. Weightmaps. No JCMs.

3. V4 17K LOD.  Weightmaps, JCMs, Custom morph.

4. David-3WM Final. Weightmaps, JCMs, Custom morph.

5. David-3WM Distribution. Weightmaps. No JCMs except for buttock.

6. David 3 Default.

As you can see, weightmapped rigging can be greatly improved by JCMs.

Still, weightmaps can reduce the need for JCMs or allow "weaker JCMs", so if they are available, why not use them.

Ideally, rig as good as you can without JCMs, then use them only as much as needed.