The problem as I see it, and correct me if I'm wrong, is that Halsin is the only character with an exclusive mission that you cannot complete without him. For example, if you dislike Shadowheart, you can simply choose not to recruit her in Act 1, or kill her and take the artifact and you can still fulfill her mission in Act 2. The same goes for Karlach and Mirthana. However, with Halsin, you can't. You're forced to have him standing there in the camp doing nothing to fulfill his mission. The game should provide an alternative method to cure the shadow curse without him, but it doesn't.