Here's something I posted over a year ago on this forum regarding Stoneskin II and why it has a cast time:

Quote Originally Posted by Ghishlain View Post
Given how anti-Speed Run the Dev team has been in their dungeon design, the only legitimate reason I can think of that they didn't make SS2 instant cast was to give WHMs a choice in their resource management in a speedrun environment.

"Do I burn my swiftcast on that Stoneskin II? Or perhaps I'll burn it with a Holy I'll use later down the line? Oh, and the MP cost on Stoneskin II is pretty high for four people, maybe I should save the MP for another Holy or Cure on the next pack" etc etc.

I guess you could say it was an attempt to give WHM a bit more depth in their toolkit.

Personally, I believe SS2 as a QoL tool for Raid groups, allowing groups to only wait 8 seconds (or less if the WHM burns Swiftcast) instead of 24 seconds between wipes. Kinda funny, if they allowed SS2 to only be cast in only a Full Party composition, they might've avoided some of these threads popping up. But hey, hindsight is 20/20.
Probably still quite legitimate. Annoying as heck, but I'm not holding my breath for any change to how SSII operates in the near future.