If you look closely at a good spoke,you will see it has rolled threads and a rolled radius under the head.Same holds true for bolts and fasteners.This will help spread out stress loads.Butted spokes work better because the middle parts (thinner) absorb more of the tension differences when being loaded/unloaded.As opposed to the heads and threads having to do it.It spreads the tension loads over a greater area,thus less fatigue failures.

Thanks!....Now if you'll check some graded and ungrades bolts for us.....