The bonus for having an elemental weapon is pretty huge. I thought the same thing, only with Lily. But then I made the 4* Water weapon even though it had the defense buff skill and saw the damage on my attack skills go up by about 1,000 damage each.
Yeah, but Lily has 2 nukes in her kit. Amane's 2nd skill is a HP buff, which means I'm down to 1 (though fairly fast charging) skill for damage