Kaeshi: Higanbana exists because three iaijutsu options exist, depending sen count, and Tsubame-gaeshi is intended as a follow-up to iaijutsu. That is all. Just because something exists doesn't mean it needs to be useful. Taking the example of healers, WHM has Cure 1 as Conjurer, and it's never useful as soon as Cure 2 is added to the kit, which again is not as useful as Afflatus Solace/Tetragrammaton as soon as they become available. Some skills exist to only be "traps" for people who don't understand a class, or use it in situations where they don't need to understand the class.
As for why Kaeshi:Higanbana is not useful, it comes down to resource generation opportunity cost for overall potency output. If I need to double dot something, like add phase in UCoB or DSR, it is less of a potency loss to generate (one sticker-use Higanbana)x2, than to waste a Tsubame on a second dot. Can this be fixed to make Kaeshi: Higanbana "useful"? I don't think it can with the current samurai kit and sen generation, unless they absolutely gut Kaeshi: Setsugekka's potency, which in turn affects every other situation where you use that skill instead, or make the sen generation opportunity cost much higher, which I don't see happening without ruining samurai's kit entirely --- like disallowing the same sen to be generated back-to-back, like PvP ninja's ninjutsu skills, which would absolutely destroy sam's PvE kit by at least breaking Meikyo, if not demanding changes kit-wide just to make this one niche case warrant utility.
I also think this is a non-issue, because nothing really prevents you from using Kaeshi: Higanbana, unless you actually care about situations where you have to play at least somewhat optimally, *and* have two targets --- which makes my UCoB example above meaningless since that fight has been cleared with 100+ deaths, so using Kaeshi: Higanbana once in a single phase is a not going to ruin your chances of clearing that fight.