I think this is a good point...Sophie having such a pure aura is going to be very powerful, but she first has to learn how to use her powers. I think that Joan, while her aura is less pure, has more control and can therefore manipulate it to do what she wants/needs it to, more so than Sophie at this point. I believe that in time, Sophie will be able to use her aura in ways Joan can not.
And I think Spacecadet's point that Joan may be trying to gain information from or about the Witch by helping Sophie is interesting. Maybe it gives some credence to Germain being the Necromancer...maybe Germain and Joan are not all they appear to be....I a not sure they are on the "other side" but I do think there is a lot we don't know about their motivations, especially since Joan was able to remain off the Elder's radar for so long. Why did she do this? Was it just to live a quiet life? Or was there something else behind this desire to stay hidden for so long? Maybe waiting for the right time to make a move? And if so, for what end?