This post may or may not be the complete opposite of my earlier post that generated some rather passionate comments.
Since then, things have changed a bit, especially for Frost. The main reason is the discovery that, much like how Lifebloom worked in previous eras, you can pop trinkets and time with procs to get the most AP, apply new diseases, and refresh the boosted disease ad nauseam (unless the fight is such that you can’t help your diseases falling off).
For Frost DW (if you don’ t have to provide the haste buff), you also aren’t losing much by ditching the IT glyph, which was the major disadvantage of using Glyph of Disease in previous patches. Frost DW runs in Blood Presence, and with the IT glyph frequently has an abundance of unusable procs and RP. So by using GoD, you lose the functionality of overflowing RP (for IBF and AMS) and gain significantly better disease damage and more space in your rotation.
Using the glyph comes with a caveat, however. Many fights simply don’t allow it to be used effectively, both in Ulduar and Trial of the Crusader. Basically, the glyph won’t help you if you have to keep applying diseases the standard way with Icy Touch and Plague Strike. Some examples of bad fights for the GoD are Northrend Beasts, Twin Valkyries, Faction Champions, Yogg.. the list goes on.
If you have the liberty of having two DPS specs, having one DW Frost with GoD can be very beneficial especially for sit-and-DPS fights like Vezax. Right now my secondary DPS spec is DW Frost with Endless Winter for free interrupts that I use for Faction Champions and other interrupt fights. If you’re not an off-spec tank, don’t be afraid to use one general one and another very specialized one for certain situations. My primary spec is Blood, which is good at basically everything but interrupting, since losing 20 RP at crucial moments in your rotation is frustrating (due to the Death Strike glyph and the large amount of RP that DRW uses up).
P.S. There’s a new, updated thread for Unholy over at EJ which I encourage you to check out if you’re interested in that sort of thing.