This is a mechanic the game forces you to use at least twice to progress, but also gives you pop-ups in-game 3 or 4 times reminding you of the mechanic. The issue is the SSP takes FOREVER to fill up; so you don't use it enough for it to be a relevant tactic.

I know this because I barely remembered it's existence once I'd gotten to Transendia. Often times the game will through multiple waves of enemies at you in near death fights. I'm pretty sure they expected you to use Spirit Strikes, but the bar fills so slowly and you never know how many enemy waves are coming or if the next encounter will be worse. I just kept holding off using it.

Well, I am glad to be first :D

You literally cannot go through the game without using spirit strike, the first time you are literally forced to use it against the ogre boss. What about the multiple other times when you get new spirit strikes, they all cast automatically. You might be the only player who did not use this ability throughout the game somehow.