I got lucky with a roll and found a +10 Battle Princess Ring. At level 40 it has +1670 atk and no def.

This is arguably better for filler equipment on whaleship girls than event jewelry. An event ring has a total power just a little higher than 1670 but has stats distributed to defense. But the whaleship bonus due to weakness is only applied to attack. Excepting the girls that actually get used in your first (maybe second) squad that could use defense, favoring atk over defense seems to make sense.