NURBS THINK RUBBERY (organic) . . . . POLYS THINK SOLID (non-organic)

When animating WITHOUT painting weights you can vividly see the difference. A poly will crease at it's edges where a nurb "does not crease at its isoparms"

TO SEE IT: Create a nurbs cylinder scale up in "Y" to about 6.
In inputs: set sections & spans to 8 & 8. In skeleton > joint tool Place about 6 clicks top to bottom in your SIDE view, > enter, have skeleton selected, shift select cylinder. Go to skin > bind skin & ridged bind. Select some middle bones & pull.
Move it out of the way in all views & do the same for a poly cylinder, in inputs you change subdaxis & subdheight to 8 & 8. You will see the creasing.

Smooth bind would give better results for a poly. Try it. But when you give extream bends with edges in the way you will still notice a slight kink. Painting weights will help here.