For a tank, never. For DPS? When you're trying to swap all that extra accuracy for an increase in DPS output. Maybe a little more crit, a little more det, or a little more spell speed. Or maybe even when you need more accuracy because you're not meeting the accuracy reqs. For a healer? Don't think you really need to swap back to i100 for anything, but maybe just when you want more crit as a scholar, or more spell speed as a WHM. You're only losing out on like, 5-10 extra heal output per cures and physicks anyways, by going down 10 ilv on 1 gear piece (if both gear pieces don't have DET).
