When should WHM use Stoneskin I?

[Spoekes]

I actually got it from here http://www.ffxivinfo.com/class/conjurer.php which Cure III and Stoneskin say 399MP at level 50.
Stoneskin II is 532MP and applied to X many players in the party, so if you have 4 players it's as efficient as one proactive Cure I (4 x 133MP) on each of them, and if you have 8 players it's twice as efficient. But you can't cast it during battle so it's moot.

Comparing Stoneskin II (multi target) with cure I (single target) is a bad idea to begin with.
It looks much different if you compare it to a group heal like medica.

Imo, you should only use Stoneskin:
a) pre pull buff
b) full party content with mechanics/big hits that might kill someone.

Situation b) is not only for tanks. I.e it may also safe the life of a low max-HP (maybe ressed) partymember on primals signature moves.
In almost every other situation a heal is more efficient than Stoneskin.