I play mainly WHM and BLM in pvp and honestly the spellcasting interruption is pretty annoying when taking damage. The PVP abilities and surecast only last so long. After those are down you are pretty much fucked if you don't kite. They should honestly make spell push back, where if you take damage constantly your spell will take longer to finish. That would change a lot of the caster issues in PVP imo.