When talking about the top 8 at the end of 2013, how is current standing relevant?
It does not matter than Djokovic is X points ahead of Murray and Y ahead of Nadal or that Nadal has nothing to defend in the second half of the year.
At the end of year, it is only the points you have earned in 2013
Yup, defending is only relevant to establish whether anyone can capture #1 early or mid-year. At the end of the year is irrelevant of course.