Why? Mana Shield drains mana per hit. It would have to do that; otherwise, the unit would be invulnerable.
If the unit runs out of mana, then what? Would the shield loose its effect until the unit gets some mana back?
This could also interfere with spell casting. The unit wouldn't be able to reserve mana for a powerful spell. In a way, it wuold be at the mercy of its enemies.
~The_Raven