Imaginary numbers used by a magus not boosted by infinite magical energy like the grail isn't very useful. Sakura could only intuitively use it to create monsters, but without the grail's support the monsters are like the size of her hand. Also elements are fluid things and aren't strictly defined into the traditional five. There are some cultures that use the 12 zodiacs as 12 separate elements. Eastern magi use different elements than do Western magi use, and so on. It's possible that this void element isn't the same thing as tapping into the root.