From what I've read, Hagakure should be used pretty much on cooldown. I'm confused as to the reason though, because based on the math, Hagakure actually looks like a DPS loss. Meant only to fix undesired combinations of Sen (like running into an AoE fight with 3 Sen, or a boss fight with 2).
A Midare (3 Sen) has a potency of 720.
A Hagakure (3 Sen) grants 60 Kenki.
Shinten is the main converter of Kenki > DPS.
60 Kenki is 80% of the way to 3 Shinten, which would have a combined potency of 900.
80% of 900 is 720, identical to a Midare.
Except that Midare has a cast time of 2.5s, and 3 Shinten would occupy 3s, making it slower and thus less DPS.
As far as other uses of Kenki, the normal combo rotation grants enough to use Kaiten on every Iajutsu anyway, plus the occasional Shinten.

Am I missing something here?