I bought the runes at one point because I wanted “Furious Demise” to increase to 6 seconds of fury instead of 5. That way I could use the trait “Close to Death” which reduces the DS’s CD and could pop in and out of DS and get the fury boost and crit like crazy without putting all my points in the Curses trait line.
It doesn’t increase Furious Demise’s effect by 1 second though, so there’s a hang time for me before I can effectively attack again.
I think this is a bug obviously. The runes give me 30% fury duration increase, but it doesn’t affect said trait.