Er. Ok, yes a stoneskin in the middle of a fight during a long aoe works. But if it hits EVERYONE a good healer will stoneskin II. If it's a specific area/target, a good tank will gtfo of the aoe. Not put stoneskin on. If it's mid fight, there's the chance you get interrupted too. A generous paladin can reapply protect to a downed dps during a fight. But protect is not top priority. It is a little boost.
As for cross class, second wind and internal release are great for WAR. The cross class is useful. DRK gets both tank crosses. PLD gets redundancies or useless cross from conjurer