@moonlight00 : I think it's been rather obvious that she isn't 'useless'. Just that the question is what made her unable to use those magic.
Some possibilities from what we've seen includes:
1. She lacked her catalyst (the staff like her mother/cover shown)
2. The magic being taught were not compatible with her (like a certain water mage being in a family teaching fire magic)
3. Her power isn't usable until certain age (i.e. now/soon)