Smooth skinning requires more effort than rigid skinning because a single vertex/CV can be affected by multiple joints, thus painting those weights takes time. Also flexors are used in rigid skinning where with smooth skinning you can use smooth skin deformers (i.e. place a sphere under the skin and scale it when the arm bends to simulate muscle bulge) The great part about using these smooth skin deformers is you can have multiple pieces of geometry which can mimic actual muscle groups. Again it's a lot of extra work and should only reall be used when precise detail work is necessary.
Jeff