CSS3 Selectors question

Can anyone explain the reason for the restriction (Section 7):
  Pseudo-elements may only appear once in the sequence of simple
  selectors that represents the subjects of the selector.
I can understand the reason for the rule that a pseudo-element must be 
the last part of a sequence of simple selectors. However, I can't 
understand why a selector such as:
  *::selection  *::first-line
should be invalid other than arbitrariness.