https://na.finalfantasyxiv.com/jobguide/ninja/

Right off the bat you should be able to see that Armor Crush's combo potency is a whopping 140 potency higher than Huraijin's 200 potency, and 200 potency higher when executed from the flank.

I'm not some Ninja guru, but the only reason I could see someone using Huraijin is if they just died and need to reapply Huton asap or after a phase transition/boss coming back from being unable to target them.