Spell speed is the weakest substat for SMN. Generally speaking, the more a job relies on oGCD for big damage, the less desirable spell speed becomes, unless more speed also means more oGCDs (SAM).
On SMN, you want to minimize spell speed and maximize crit. A little sps won't kill you, but on SMN it has a very limited utility. Whatever guide is suggesting you stack sps on SMN, throw it away.