...way out on the corner of fucked up and evil...

I feel confident in saying that including such a scene would have raised BB from "totally awesome" to "head-explodingly fantastic". They wouldn't have even had to justify it; Batman could just be like, *nailgun*, and Katie Holmes would be all, *iz Ded* and the Scarecrow would be like, "Wtf?" and Batman would be all, *shrugs* and then they get right on with the rest of the film. Because seriously, she contributes that little to the story. She's pointless even by the normal standards of Pointless Love Interests.