Pretty simple really, it makes it open for caster retal builds to help them survive. Used on my Ba’al of Shit Amorum set Sentinel which helped use 2 Bladed Plating components and increased Freeze resist, and if SF procced on hits it would be far more tanky.
In Ascendant that is 
Ultimate it’s good enough, but yeah, the increase in damage in Ascendant would make changing SF to proc on hit (like I swear it used to do) help a lot, since that set blocks the offhand slot. Mind you, rather fun to drop big DoT’s and run away, but sometimes mistakes happen. Especially when gumbies like me art tired or had to take 1/2 a zopiclone.
Let’s imagine:
Stone Form Proc On Hit → Offhand builds use it → Offhand builds usually have decent CDR → Continuous Stone Form → Stone Form being abused → Stone Form got nerfed → Shield builds suffered.
So no to Stone Form Proc On Hit without requiring a shield.
But I agree increased damage in Ascendant pushes the need of better tank. So if you suggested higher damage absorption on Phoenix, I’ll agree.
Eh, I model that as not working like you think, because stacking CD% on retal builds usually comes at the cost of flat and % retal and both exclusive skills that do CD% don’t increase retal either. Besides, a shield is still a major key to tankiness because blocking damage is more powerful than absorb due to the shear amount of damage in Ultimate, never mind Ascendant mode.
But yeah, some more absorb on Phoenix would be very, very nice for elemental/acid retal builds.