I never did figure out why they didn't have Beatrix join the party instead of Amarant. She could easily have been fit into that role in the story, and most people already would've liked her as a character.

The only reason I can think of is that they were desperate to have each character be a different class. Which they'd already buggered up with Eiko for story reasons. But Amarant's class is hardly anything at all. Doesn't seem like much was gained.