Edit (04/10): It has been brought to my attention that Ignite may, in fact, have become a smart spell. I'll have to spend some time looking into this, so, uh, the following post exists so you can mock my terrible math ^.^
Ignite is a Fire talent in the second tier that causes your critical strikes from Fire (and Frostfire) spells to burn the target for an additional 8/16/24/32/40% of the damage dealt over 4 seconds. It ticks for 20% each tick, 2 seconds apart. This is a sizable contribution to overall damage and dps, since it essentially adds an additional 40% crit strike damage bonus (if it ticks for the full 4 seconds).
However, here is the issue. Ignite isn't a smart spell. If you crit with one spell, and then within 4 seconds crit with another spell, the ignite from the second spell will overwrite the first ignite even if the first ignite was greater. I think we could all agree that this would cost dps, but let's explore this a bit further.
Now, a disclaimer: these numbers are all based on older WWS logs, since I respecced Arcane last week. I went back to our last one-night, full clear for the data. This WWS report I'm using is from 3 weeks ago, so the numbers are lower than they are now, owing to different gear and a superior spec now.
Here's what my damage table looked like for all boss fights (no trash):
Ignite is a huge part of my overall damage. It outdid Living Bomb, which is usually considered the key dps component of Frostfire and Fire specs. While the overall damage is a bit inflated by Thaddius (I can't really crit for 26k) looking back to other runs where we didn't get to Thaddius, the breakdown is about the same.
The pyroblasts are purely from Hot Streak procs. I'd never waste time on the 5 second cast. And I apparently crit a lot with pyroblast, which means I cast Ignite a lot from pyroblast. Let's do some number crunching:
Hot Streak procs from Fireball, Fireblast, Living Bomb (the bomb part, not the dot), Frostfire Bolt and Scorch. For these spells, I had a total of 433 spell hits, of which 255 were crits - about a 59% overall crit rate. I procced Hot Streak 188 times through the whole instance - but bear in mind several factors, not the least of which is Lotheb. Compare the proc rate - 188 - with the hit rate - 79. I wasted a lot of Hot Streaks.
Normally, if I proc a Hot Streak, I hold on to it. If Living Bomb is about to go off, I prioritize letting LB finishing before I toss of the Pyroblast. Why? Ignite munching!
I assume a spell is going to crit. If Living Bomb goes off and crits, as per the above WWS, it will do a (slightly inflated) average of 2085, with an average crit of 3656. The Ignite from this Living Bomb would be 1462 damage over 4 seconds - and extra 365.5 dps while that Ignite is active.
Pyroblast hits for an average crit of 9399, which would make the Ignite bonus 3760 over 4 seconds, or an extra 940 dps. That is a huge increase over the Ignite bonus from Living Bomb.
All of this assumes that both the Pyroblast and the Living Bomb will crit, but nonetheless, this would result in a 574.5 dps loss if the Ignite from Living Bomb overwrites the Pyroblast. As such, I usually hold on to my Hot Streak procs until after Living Bomb ticks (unless it's way early in the Living Bomb duration).
Sometimes, I proc a Hot Streak before I've cast the first Hot Streak. This is also a massive loss in dps. Not only are we losing the Ingite (if it crits), I've also lost the 4888 average hit of the Pyroblast. I never ever intend to do this, but sometimes I proc twice in a row, or I lose it waiting for Living Bomb. I rarely cast the Hot Streaks when I should have - I cast barely half of the procs I got (I got 157 procs on bosses that weren't Loatheb). I was a terrible Frostfire mage :D
All in all, I figure it's best to hold on to that Hot Streak until LB is up, but I absolutely welcome feedback.