While I might say it not clearly due to text limit in above post. I might as well give more explanation.

1st: Stoneskin, is never meant for absorb damage for Tank in the past, which I believe will be also in the future. There are only few occasions you should cast stoneskin on Tank, which are
1) Before Pulling Boss / Pack of Mobs .
2) You have nothing to do, and Tank doesn't take much damage.
3) When you have enmity problems.

2nd: Stoneskin, usually cast on other, even us, WHM @lv50 hp~ 3.5k can absorb for 630 damage. and this 630 <> 0. You can take hit more 630 dmg as well as no interruption while casting spell. I won't call this 'nothing' and serves no purpose.
3rd: OK, Warrior has the biggest HP pool, esp at lv50 which hp ~5.5k - 6k. So what? PLD might not as high but stoneskin on 4k+ hp is also not 0 dmg. And PLD take much less damage from all of his defensive skill. I won't call this 'nothing' and serves no purpose also.