Raiding, since that's what most people are talking about/worried about, tanks should already know how to handle multi-mob/multi-tank situations, so I don't see where "holding up the raid" would be an issue. I'd also hope that players are disciplined enough by the time you get into a raid scenario, or again, I don't see a Glyph as being something that can fix the issue.
Now if in Wrath we start to see a lot of encounters where Avenger's Shield causes problems, I'd happily change my tune on this Glyph and give it a go :) Right now from all the content that I have tanked I do not see this Glyph as being necessary in many, if any at all, situations when you take into consideration that you would need to have it on the entire raid (or cause long pauses so you can find a Lexicon). There's tactics that tanks should have learned leveling up and going through new content that are quite effective for handling pulls in which you have to be careful of CC (remember when AS hit CCd targets?) or that you wanted multiple tanks.