Quote Originally Posted by Rilifane View Post
You don't get any additional value out of SpS because you have neither casts you regulary hardcast that have a cast time that's longer than your recast time = GCD nor dots, though the effect on dots isn't huge. More like a nice little cherry on top if you're going for it anyway. Moreover your melee combo skills don't scale with SpS at all which further invalidates SpS.
But wether a spell is instant or not doesn't matter as SpS lowers the GCD of affected spells aswell, so having instant casts doesn't make SpS worthless and saying that it doesn't count for the dualcasted spell is wrong. It affects both your cast time AND your GCD. That's something that should be kept in mind when trying to understand the different substats and their value for each job.

For BLM it's a fantastic stat since Fire IV has a longer cast time than your GCD plus it gives more room to add and switch around casts without the risk of dropping Enochion.
While the effect is subtle, it also helps with mobility. Chances are higher you can finish your cast before having to slidecasting away plus more casts per time means more slidecast windows to cover more distance.
But BLM is the only caster actively stacking SpS. SMN and RDM have the same priority and you can use the same gear for both in case you become interested in SMN.
This is my understanding of it as well. Spellspeed still affects the GCD of instant cast spells, but RDM also has weaponskills that are not spells and not affected by spellspeed. Paladin has the same problem with the reverse proportion of weaponskills and spells.