Wren's not trying to be difficult, but she is thinking about keeping her skin on her rather shapely form and her heart beating. She has had enough experience with area effecting spells that she's wary of anything that empeeds the good gys as well as the bad guys... and there's a very valid point made. They aren't being paid to kill the bad guys, just find them, with express orders that they can defend themselves, not act on their own. Becoming a wight put in chains just isn't appealing to her.