Why are natural levels preferable to arbitrary levels?

Why are natural levels preferable to arbitrary levels?
 
  a. Arbitrary levels can potentially jumble together artifacts that come from different natural strata and thus different periods of time.
  b. The depth of natural levels is determined by statistical sampling strategies, while arbitrary levels are chosen subjectively.
  c. Arbitrary levels follow the natural stratigraphy, which may not be able to distinguish between occupational surfaces.
  d. Natural levels are much simpler and faster to excavate than arbitrary levels.
  e. Natural levels are the method taught in New World archaeology