What for?
PvP daggers are currently equal to pve weapons:
T3 PVP:
98.25 DPS
+200 Health
+180 Stamina
+4.5 Natural Stamina Regen
+3.75 OOC Health Regen
+13.5 OOC Stamina Regen
+32.5 Evade Rating
+50 Critical Rating
+60 Critical Damage Rating
+41 PvP Hit Rating
T4 PVE
98.41 DPS
+200 Stamina
+5 Natural Stamina Regen
+15 OOC Stamina Regen
+50 Evade Rating
+50 Hit Rating
+61 Critical Rating
+73 Critical Damage Rating
In current set up Dagon's Spine is just a gimmick
Dragons Spine T3 PVE crafted (fully gemmed
http://3.bp.blogspot.com/-9Z5JmKHrcO...ScreenClip.png)
88.08 DPS +3.83 DPS = 91,91 DPS
+55 Critical Rating
+116 Critical Damage Rating
+55 Offhand Rating
+ 41 fatality rating
As you can see T4 and T3 PVP are pretty much the same with the exception that T4 gives more critical and T3 PVP gives more HP anr HP regen. Dagons Spine is far behind and Bel's Mirth proc doesn't go off enough to justify rather substantial DPS trade off (Bel's Mirth is designed to be a DPS boost for sustained damage being dealt in raid raid environment).
My suggestion is: Get PVP T3 for PVP.
The damage and criticals difference between full T3 pvp and T4 PVE is rather small and the sheer survivality of T3 PVP +protection and +HP buffs outclasses PVE gear by far. Extra 50 constitution that you get from 2 x pvp daggers complements whole PVP set and thus I advise not to mix it with PVE gear.