Spell Monsters cannot be activated on your opponent's turn, making them unable to perform the "Emergency" Defense that Trap Monsters can. They can instead provide an immediate source of Tribute Fodder, much like some cards do already, like "Fiend's Sanctuary".
As shown in Yu-Gi-Oh! GX they can be used as Fusion Material, as Jaden was able to fuse his "Elemental HERO Neos" with "The Rival's Name" (which was being treated as "Rainbow Dragon") to create "Rainbow Neos".
In Yu-Gi-Oh! ZEXAL and ARC-V, they are shown to be able to be used as Xyz Material, with Fortuno overlaying "Xyz Plant" with "Rainbow Shaman" to Summon "Number 11: Big Eye" in ZEXAL, and Yuto overlaying two copies of "The Phantom Knights of Shadow Veil" to Summon "Dark Rebellion Xyz Dragon" in ARC-V.