So just what was Belial's plan there?

[Elric of Grans]

I read that the idea was the player was supposed to really care about and for Leah, so when she gets screwed over the emotional impact is larger. Trouble is, you never grow any emotional connections with her. Part of it is her poorly written dialogue; part of it comes from how little interaction you have with her over the game. If she followed you all the time and had considerably more banter with you and your companion (especially along the lines of her dreams and ambitions), then the twist would have been meaningful!