I don't really understand it. Better use that 20 seconds to tell those ninjas about Kassatsu.
Using Kassatsu on Suiton is a pretty bad DPS loss. It's like a BRD continuing to use Barrage on Heavy Shot even after he gains Empyreal Arrow; it's a waste of the skill's potential.

Please don't insult your NIN's intelligence by assuming he doesn't know about his own abilities >_>