Originally Posted by
Wingwraith
You're missing the fact that you already used Festering Strike to get that death rune.
As you are aware, SS does more damage than Fest Strike. So, to maximize dps you want to prioritize SS over Fest Strike. What that means is that you only want to use Fest Strike when you can not use SS, ie. only use Fest Strike on blood+frost pairs.
If you did use Fest Strike with a Death Rune, you would greatly reduce your total number of SS's throughout the course of a fight resulting in much lower dps.
Does that make more sense?