Looking at the Ninjutsu toolkit we have

Fast Single Target Hit
Med Single Target Hit
Sneak Enabler
Med AoE Hit
AoE DoT
Attack Speed Boost
Single Target Bind <- The one in question.

When we take the Ninja's other ability and their tools into consideration, it's hard to justify giving them something else, but Hyoton is effectively worthless. Fuma/Raiton could use some competition for its spot.

Hyoton: Deals X Ice Potency damage. For the next 30 seconds, the Ninja's Ninki generation is increased by 15%
or
The Ninja's next magic damage source benefits from Poison.