Added a BDFL justification to why there's no optional pad argument.

pep-0201.txt

@@ -188,7 +188,10 @@ BDFL Pronouncements
       argument sequences were not the same length.  This is similar
       behavior to the map(None, ...) semantics except that the user
       would be able to specify pad object.  This has been rejected by
-      the BDFL in favor of always truncating to the shortest sequence.
+      the BDFL in favor of always truncating to the shortest sequence,
+      because of the KISS principle.  If there's a true need, it is
+      easier to add later.  If it is not needed, it would still be
+      impossible to delete it in the future.
 
     - Lazy evaluation.  An earlier version of this PEP proposed that
       zip() return a built-in object that performed lazy evaluation