Its not that Scorch is useful on every momentIn the past I've always known scorch as pointless to cast from what guides have told me anyway. So is it actually useful now?
it is that During Combustion you have 100% Crit for 10 Sec
and you want to get as many Pyroblasts as you can during those 10 Sec
Ideally you want to only Cast Fireblast (off GCD) and Pyroblast!
Obviously this is not possible ause you will have only 2 Fireblast procs (this is way Flame On is taken, to get another 2 Fireblasts for the Combustion time)
Coming Legion we will have Pheonix Flame as well, which is an Instant Spell
3 Pheonix Flames + 4 Fireblasts (with Flame On) + 8 Pyroblast! proc will be 12 GCDs so it should usually be all the casting you will be able to do
But we do not have Pheonix Flames today
so we must have filler, Scorch is 25% faster then the Fireball so people believe it will be better
I have no idea i the Sims agree or not
but it sounds correct, 4 Fireblast + Flame On + 5 Pyro! is 6 GCD , if you will add Fireball+Pyro! while the scorch might let you Scorch-Pyro!-Scorch and you will have a Pyro! for when the Combustion ends (or with enough Haste even inside the Combustion)