Well, Bob makes a good point in that the side plot, if done right, could be quite interesting.
You could also make her a strategist, or tactician. No matter how strong the team is, if they just run ahead madly into the dungeons then it'd just be chaos.
Another part of it is that diversity of character is always a good thing.
As well as the fact that it wouldn't be much of a story if they never had any difficulties to overcome, and with multiple character types you have far more options for scenarios and how they overcome... or don't.