Try this one out: Lets say u have 0,1 sec less cast time on fire then i have, that means u need a few casts to actually get ahead of me.

we both start with the same rotation. if we both get the interrrupt at the same time, lets say in 2nd fire rotation, ur mid way tho cast fire and im at the start of casting it, then ur ss bonus wasnt enough to gave u +1 fire cast compared to me.
that means we both had the same amount of spells, but my spells where hitting harder cause of the extra INT and we start again and ur bonus of beeing half a spell ahead is gone .

if u get that bonus form ss all the fight long and that means u can cast without getting interrupted, u can say its the strongest stat for BLM.