Even if she was able to improve her physical state by transforming into a more powerful shape and size using the body of someone else, it was still just a transformation.
It still cost her Life Energy that could be used in other areas during a fight so it was not wise to waste her Life Energy on transforming unless it was needed.
A fatal weakness of her being able to transform was that she needed to learn how to properly use her new body type.
Not only that but it was still a transformation that could only be kept up for a while.
There would be a time when she would eventually transform back to her original form and that would restore her to her original physical state.
If her physical state was very poor at that time then it could become a very fatal weakness for her in the future.
