Because if we don't take this chance to kill him (and/or Spike come to think of it) early enough, then that dedicated fanbase devoted to them will shove them to the very top. Do you want the very end to be the inevictable Giles/Spike face-off? No? Then help kill them now while we have a chance!
Vote Giles, Spike and Andrew to die!
Vote Faith, Xander and Wesley to kill them!
You know it makes sense.