There are so many targets and so little time.Do you know who else liked keeping lists, Wargfoot...?
More seriously, why bother keeping a list? Blow up everything you can or die trying. Mercy can always be ad hoc granted on a whim for things such as witty comms or "oh yeah that's my buddy maybe I shouldn't blow him up". Keeping lists is so tedious. Bad enough we need to mat farm.
Imagine how foolish I'd feel if I'd spent all my ammo blowing up Random Nobody only to have Kari fly by a few minutes later?
All the best people keep lists. Nixon kept lists. Q.E.D.