Graendal being alive makes it possible for her to reveal herself as Asmodean's assassin - well, I believe it was her, anyway. Prior to the prologue, I was getting nervous about the prospect of 'dead Graendal' and all it would bode for this mystery of the series.

I would go as far to say that it might be the reason that she's still alive. As someone said, Graendal seems to be about infodump and BS went to some effort to prevent her from dying whereas Aran'gar went down the hole for the second time without much fuss.

Of course, I'd like Graendal to play a much bigger role still, and I suspect she might be the second last to fall before Moridin (which would nice since she was the second to join the Shadow). Until then I'm hoping she muses on how she snuffed Aran'gar and links it to her other memorable murders.