edit: I also heard from one of my ret friends that even if you have 24 expertise, the exorcism glyph is better than the expertise one, although I do not personally have the data to back that up.
The intial suggestion was if you were able to benefit from at least half of the SoV glyph, it was worth it over exorcism. Never checked back on any updates from what I originally read. So 21-26 expertise the choice would have been exo. Below 21 you would use SoV. I believe (could be wrong) this was assumed you were fighting Humanoids as well so in ICC you can probably get away with sacrificing more expertise for the exo glyph due to all the undead.
Did a post in the EU-forums asking about that some minutes ago...
Seeing as I did 50+ runs of Marrowgar Trash in 10man, I have quite a big grasp of how it works. Well, I SHOULD have... but honestly, I have no idea.
I could see some things...
- they seem to dislike trolls... especially shamans. A LOT. the chance to awaken them seems to be way higher when you're a troll.
- the first two Wards sometimes awaken as soon as you kill the 3 "The Damned"-group and sometimes stay petrified until all other Trash is dead. Same goes for the last two Wards. (although they tend to stay petrified and rather rarely activate while the last group is not already totally killed... only happend once or twice)
- if you're at the last two Wards and they're still petrified, try going near them or have someone between them and the gate to Marrowgar. It's hard to describe... but if you think of the line between the instance portal and marrowgar, just stay a tad on it's right/left side and there you should have a bigger chance to trigger them. (~5-10 yards away from the invisible line in the middle)
Other than that... just can't really tell... it's soooo damn random.... like Deep Breath >_>
fun fact: a hunter can pull them by puting a trap below his feet.