From the feedback I've gotten so far, I would like to add:
1. I don't want to argue Pharamere's is BiS because of consistency VS randomness. However, I value consistency definitly as a DPS increase. Examples:
1.A) Singeltarget Pyromaniac / MI / FO / UM / CiS.

Worth to mention the error is 500 dps on 10k iterations. I should run more iterations for some itemlevels, but on i.e. 920 itemlevel you do see that Whispers outperforms Erratic on average. Erratic is 12 RPPM, whispers 0.7 RPPM. You should have seen reports that even on 2-3 minute fights there was no Whispers in the dark procc. This is possible, this is rng. Thats why I value Erratic > Whispers for Singeltarget. And if you would take a look at what other casters get out of whispers trinket then I highly recommend to pass for this trinket untill every other class in your raid has it. Should mention that if you just want DPS-rankings then whispers in the dark is your padding trinket #1.
2. I don't want to explain why this trinket is so OP, because I can not do that without wasting tons of time. But I can give you the idea behind it. And I really can't give you a picture describing current mage trinkets. The closest one I can do is:
On high adduptime:
Fury > Stargate > Pharemere's > Erratic >= Whispers
On no add uptime:
Erratic >= Whispers > Fury >= Stargate > Pharemere's
On 1+ of targets but quite low add uptime. This is where I would fit in almost every raidboss in Nighthold.
Pharemere's >>>>> Erratic >= Fury >= Stargate >= Whispers [2nd trinket depends on talents. Other classes get way more out of Whispers. You probably want Erratic, Stargate or Fury.]
For mythics: Fury > Pharemere's > Stargate > Erratic >= Whispers.
Examples:
2.A) Simple 2 target simulations, 100% add uptime.

The error is still 500 dps on 10k iterations. I could make the same RNG statement as above, it still fits. However, if the add uptime is lower but still lines up with On-Use-trinket you should realize that Pharamere's will pull ahead. The raid simulation which I do need to sim that Pharamere is decent is an add-fight. Not Singeltarget, not a council like fight where you've 3 targets the whole time.
Furthermore stuff like desynced adds from Pharameres or no adds during combustion are things that highly devalue the on use trinket. Do you want to use it during Combustion or delay it to adds? Do you want to use it on cooldown or delay it to adds? Mostly delaying it for adds is the right choice for that one.
What if you don't run MI? RoP is a 40% buff to this trinkets damage portion. This is huge.
2.B) To be honest the easiest way to match the value of this trinket is the following approach:
-
http://www.wowhead.com/item=140801/fury ... =1497:3514" target="_blank is roughly equally designed. ~1000 secondary stat + Damage stuff.
- If
http://www.wowhead.com/item=140800/phar ... =1497:3514" target="_blank damage portion does outperform Fury's damage portion it is probably better. (There is still the haste / mastery thing, but lets ignore it for now)
- Fury sims highest for 2 or more target fights with 100% adduptime. If Pharamere beats it, it is obviously better.
- Whispers in the dark sims highest for 1 target.
Correctly mathing this out is anoying. Here comes napkin math! Way easier but not correct.
Looking into simcraft, this is fury's dps for Singeltarget Pyromaniac / MI / FO / UM / CiS.

~29k dps from procc. Worth to mention that a 920 itemlevel fury has a value of ~40k dps. Furthermore the damage of fury scales with targets, cause it is an AoE Damage. So If I have 2 targets it does ~58k dps.
So to describe Fury's damage I can use:
29000 * targets * uptime in %.
For Pharamere's I could do the same. Basically, the result should be: Pharamere's damage * class multipliers * crit multiplier. Having Combustion for every 2nd Pharamere would really buff this one, so I simply turned combustion off. From Simcraft I get 20000 dps * targets.
29000 * targets * uptime in % < 20000 * targets => uptime < 69%. Shortly, on equal targets I need 70% add uptime to make fury of the burning Sky outperform the on use trinket if the on use trinket is used on cooldown but outside combustion.
Same applies to Whispers for singeltarget. For Whispers in the dark I can compare the difference between fury and the trinket and then create a buffed fury that matches it so I can describe the value of Whispers in the dark with. This is simcraft result with:

Close enough, so I get this: 39000* targets * uptime in % as a function to describe Whispers in the dark. [for 1 target atleast]
39000* targets * uptime in % < 20000 => uptime < 51%. Shortly, on 1 target I need 51% add uptime to Whispers in the dark outperform the on use trinket if the on use trinket is used on cooldown but outside combustion.
Should mention that burning sky should be the best trinket for 2 target+ council fights so basically:
Pharamere's BiS if you can use it on cooldown with:
- Singeltarget adduptime <51%
- Multitarget adduptime <69%
This is complete napkin math, but should give a ROUGH idea where the trinket is. Personally, I don't trust those results. I find myself in situations where I don't use it on cooldown, instead I keep it for adds. But I do trust that I can easily describe Fury of the burning sky and Whispers in the dark with the above functions quite decently. Taking those functions I can easily look into a log and compare the amount of damage MY SPECIFIC use of Pharamere's does compared to other trinkets. Still have to judge if the damage which it does is usefull.
2.C) This is probably the best approach:
Scorp - adds appear way more often
Anomaly - only one add, often not stacked with the boss. Or to wait to pad on the small ones? Half-check.
Trillax - ST
Aluriel - check
Krosus - ST
Tich - way more often
Etraeus - ST (not stacked or padding on little ones, which is not useful at all)
Botan - adds are up more than 66% of the fight.
Elisande - one add? Half-check.
Guldan - one add and 3 at once for 1 time and a lucky snipe on eyes? Lets say check.
Total 3/10.
For me that list looks like:
Scorp - Trash mob.
Anomaly - Trash mob.
Trillax - Trash mob if you find the dispell key.
Aluriel - check
Krosus - ST, if you struggle with adds and I bet you will check.
Tich - If you struggle with BATS and I bet you will then Check. Add DPS outside of bats hardly matters from my PoV. Otherwise this is ST.
Etraeus - ST
Botan - adds are up more than 66% of the fight. Difficult to judge. Fury may be competitive, but probably check.
Elisande - Check.
Guldan - one add and 3 at once for 1 time and a lucky snipe on eyes? Not sure about this huge delays on the trinket.
5/7 not counting guldan, but probably gonna be 6/7. I highly recommend to think of Pharamere's as another Dragonsbreath legendary. But as Lahrast mentioned, it depends if you need AoE DPS.
3.) Fury / Stargate is also underrated on the Graph from Binkenstein. Mostly because they scale better with other talents / legendaries / haste values.
my 910 fury sims higher on ST than the standard 900 metronome because with belt/bracers and RoP mastery is equal to haste and vers (crit > int > vers=mastery=haste to be precise), even with 2p bonus wich i have
This. Highly recommend to sim for the question fury > stargate > whispers > metronome.