While most of the Stygian Guard is magic focused, these Sirens form a melee contingent of the clan. I've still helped them to enchant their blades to reap the benefits of the Titan's power.
Runestone is effective on units who have the Incant affix. You will already want to regularly cast your spells on a particular floor (with your Incant units). Having Runestone on a unit (especially front line units) with Incant will gain an additional bonus. Though it can be argued that the Siren of the Sea is or isn't a front line unit, during the Relentless phases of the boss fight, the Armor provided by Runestone still allows her to attack more times and deal more damage.
Frenzystone is effective on units with the potential to obtain a high Attack value, effectively double the damage dealt (which could mean the death of two enemies, rather than one). Doubling a value of 100 Attack is far more effective than doubling 10 Attack.