I've been having an argument about Glyph of Reflective Shield, and instead of de-railing the thread, I figured I'd just come and post here and see what H2P thinks of my logic.
I agree that Glyph of Mind Flay is (in almost all fights) the go to glyph.
After that I would say that Glyph of Reflective Shield, Glyph of Fade, Glyph of Inner Sanctum and Glyph of Inner Fire are your primary defensive glyphs. Of those, Power Word Shield generally does the most to actually keep you alive; since it's the only one that costs you dps time, this makes sense.
Then you get.. other glyphs, which have varying purposes. For instance, the glyph that reduces dispersion cooldown by 12.5%. For myself, I consider Dispersion something that should not be used if at all possible in a pve setting (due to the dps cost), unless you are doing something like solo soaking Static Shocks. Those places that you DO need dispersion, are generally something that you can plan ahead for, and I can't think of one where being able to use dispersion 15 seconds faster actually matters. (An exception could be weak healers on Norushen I suppose, but your healers would have to be at a very specific level of ability in order for it to matter.)
Another one that could actually compete is Vampiric Embrace, so that you get front loaded healing. I suppose I tend to downrank that one due to my own guild, but I generally find, in most cases where a big healing cooldown is needed, its often needed for more than 10 seconds. And in most cases when a big healing cooldown is needed, usually the healers can ALMOST cover the healing without it, and so you're just topping off the last bit of healing, in which case being able to heal more from vampiric embrace just increases it's over healing.
So now my glyphs look like this:
Glyph of Mind Flay + 2 of:
Inner Fire, Inner Sanctum, Reflective Shield, Fade (damage reduction), Dispersion, Vampiric Embrace.
In my opinion, Dispersion simply isn't required. Vampiric Embrace is situational depending on your healers strength. Of the other 4, either you need extra damage mitigation in an encounter, in which case, I would want the strongest one, or you don't, in which case you don't need any of them and you take Inner Sanctum for the speed boost. (As a comparison, for Inner Sanctum to provide the same damage reduction as Power Word Shield, you need to take almost 1.7 million damage in 15 seconds, which isn't as unreasonable as the fade glyph, but still a touch higher than I would expect in most cases.)
Now often people say that the numbers from Reflective Shield are too low to warrant it. But glancing at my last set of logs, our shadow priest used PW:S 13 times, and did 800k damage with reflective shield. That means she did damage with 11 of them, which, given 2 chunks of movement on that kill (for body and soul), makes sense. Of course the number is still ridiculously low in terms of overall dps. But it means for each cast of shield for damage mitigation purposes she kept dps going.
Of course, if your healers are awesome, then you take mind flay, sanctum (for the speed boost after a wipe), and dispersion (because nothing else matters); except if you're like me, you're still going to cast PW:S on yourself every so often, so why not take reflective shield so that when you are silly and forget that your healers can do everything, you still keep SOME dps going for that one second? (Since the third glyph doesn't matter anyway!)