It can't be used in combat.
It's used pre-pull, so the mana cost is irrelevant.
For the life of me, I can't figure out the purpose of the 5sec cast time other than to make the application annoying for 8 people at a time.

Consider this, the cast time on Stoneskin II is so annoying that WHMs will blow Swiftcast for an instant application just to avoid it, despite that there's really nothing lost other than 5 seconds of time.

Casting Stoneskin II is demonstrably less valuable than 5 seconds of time.

This is right up on my list of top QoL changes, right under resetting CDs for a raid wipe. Please stop wasting player time, we just want to play the game.