Actually, if you had Marnie and Waltz, then you would use both. Marnie is superior because she provides the greatest overall boost, while Waltz can provide a general boost (30% of her stats) for multiple purposes such as farming runs. With her Skill AW, it's still a flat +100 ATK/DEF for units in her range and that's a huge boost for a lot of units that can make full use of it like Vampire Hunters.

I still do plan on raising Waltz since I'll have many uses for her despite having Marnie with her Skill AW. She'll provide the damage some units need to meet higher damage thresholds because an AW Thief makes it harder for some units to kill an enemy in one hit. Waltz is a fix for this issue at a low cost of 4 points and one ranged spot.

Waltz is also a Super Saiyan.