The point of Accessioning a DS cure IV when WHM already has curagas, would be to put up a potent cureskin.
It's a defensive measure for when you can see some major AoE pain on the way shortly. But it has limited application, since there's only so many situations where you can see it coming and have time to JA up then cure. If everyone's already low on HP, you're just gonna curaga to get HP back up fast.