Fel flame is an instant cast which does some damage and will add 6 seconds onto the duration of your unstable Affliction. (Idly: I wonder how that’s effected by haste.. and by added dots as a result of haste)
Unstable Affliction has a 1.5 second cast time (assuming unglyphed) so the cast time is totally equal to the global cooldown (as you gain haste the cast time and GCD will decrease by the same amount). A full cast of UA gives you 15 seconds of the DoT.
At this time in a standard “Patchwerk fight” there is no appreciable DPS benefit to casting Fel Flame (6 additional seconds of UA) rather than casting a full Unstable Affliction (15 seconds of UA). The direct damage component doesn’t offset the additional GCD needed to fully extend UA. It’s possible that with the new 4-piece bonus this may change.
That said, as mentioned, TPTB do their theorycrafting based on a stand-and-deliver fight. If you instead consider a fight where you have to move around.. then Fel Flames gives you an additional instant you can cast while you’re moving.
Things you could be doing while moving:
– Soulburned Soul Fire
– Life Tap (if you need mana and aren’t too low on health)
– Refresh corruption (anytime, if you have to move you might as well extend this duration)
– Refresh Bane of Agony (only if it has dropped, don’t overwrite ticks)
– Refresh your Curse (if it’s in danger of dropping)
– Fel Flame (A little bit of damage + more UA)
Add to that, this makes a great “totem killer” in PVP.
/cast [target=mouseover] Fel Flame