[Shadok]

Compared to a stone wall it´s alot more expensive to cast (mana vise)
There for make it possible for both arrows and spells to pass (just have to check it won´t make it to easy to kill Ogre lords and other stuff) this way it won´t be compleately safe to hunt the big monsters, since they can still use there magic.
Or make it hard to dispell, a stone wall will always dispell but energy field will have a x% of being dispelled, that way you can use it as the "expensive" para trap version of stone wall.