All right. Hear me out first.
I'm not debating Glyph of SS's usefulness in terms of PvE. It's great for PvE. You just keep Scourge Striking your little heart out because there are set rotations in PvE.. which.. as long as the Glyph refreshes the diseases.. pretty much consists of Scourge Striking nonstop.
PvP is a different monster though. There really isn't such a thing as a rotation in PvP. You do what you can to kill something as fast as possible. And sometimes this involves more than attacking constantly. CC, silences, etc..
Most high rated Unholy DKs I have seen spec deep enough in Frost for the talent that makes Chains of Ice cause Frost Fever. It's a bit of a stretch to say that Chains of Ice will only be used once per match followed by non-stop Scourge Striking. The fact of the matter is that there will be Chains of Ice aplenty. Every time you use Chains of Ice you expend a frost rune and thus leave a hanging unholy rune (except for the "free" Chains of Ices from the PvP glove bonus, which is being done away with in the next patch).
There's not much you can do with a single Unholy rune. You can throw down an AMZ if you're spec'd for it. You can refresh Bone Shield if it's available. Other than that you're pretty much left with Plague Strike. Given how much Chains of Ice is used, and the fact that it will disease a target, doesn't this somewhat nullify the point of the Scourge Strike glyph?
There are a few other glyphs out there that I can perhaps see being more suitable. Bone Shield, AMS, Strangulate, or even the Chains of Ice glyph for some free damage.
I'm not above admitting I'm wrong, and I may be on this matter.. but I just wanted to throw the idea out there to see what people think.
(This thread assumes you have the following glyphs: Ghoul, Scourge Strike, Dark Death)