Think about it like this, each rup/evis is 10sec off killing spree's cooldown. With hwf AoC its effective cooldown is about 35 seconds. That means EVERY time you perform a damaging finisher while spree is off cooldown, it's approximately 1/3 of a killing spree usage thrown away. At 1.7 usages per minute, you'll use it ~5 times per fight. That means if you delay even 1 finisher every time that 5 times goes down to 4 or even 3 uses.