It's actually ineffecient for the WHM to use their MP on a PLD, most of the time. I don't have access to my client currently, but stoneskin costs a lot of MP for a heal that's under cure's essentially. Warriors are abit of a different story, as a stoneskin on them from a WHM is worth 1260+ HP, whereas a full DL / relic +1 WHM's cure is going to be hitting for high 900's noncrit, but of course, SS can't crit either.
A Paladin floats at rough 6kish HP at best, or 1080, not even a 100 HP increase over cure, yet it requires a sizable sum of MP more. Most Paladins are going to be around the 5.6k mark, or an effective stoneskin of 1008. Not worth the trade off of higher MP costs and lack of crit.
That being said, for a PLD who has hate established and isn't AoE tanking (Boss fights), stoneskin gives himself a 560 HP shield, which is pretty nice for a tank buff that has no recast.
EDIT to contnue: Most WHMs only use SS as a pre-shield, and if they're MP is so good they could recast it mid fight, they'd probably be better off nuking.