Form shift isn't meant to be used while you have enemies up. It's always better damage wise to do your rotation unless you have to be away from the enemy dodging an aoe or doing mechanics. This includes aoe situations. If the enemies are up long enough that you can't sustain through natural ticks and purification + invigorate, then there's a problem. Unless you have 4+ enemies there's not much need to use Arm of the destroyer anyway and that's your biggest tp consumer anyway