I feel like we're speaking a different language.
Your tank has 7000 health. You put Stoneskin on him. Now he has 8260 health.
That's actual health, even if you can only see the 7000 health on him.
That's huge. That's essentially like he gained a ton more vitality without sacrificing any other secondary stats. Damage prevented is damage never taken.
Stoneskin keeps people from being one-shot. It allows lower geared players to have higher survivability in a way that no other spell a WHM has can accomplish. And if you're really smart, you can figure out swing timers in almost any encounter and pre-plan when you will cast your Stoneskin, and when you will hold it back for later. When you're playing with effective health, you have to do pre-planning, but it is 100% worth it because the payoff far exceeds anything you can do with reactive healing.