Spirits in The Tempest

In How to Read Literature Like a Professor, Foster makes the point that authors of literature do not give characters disabilities unless there is a reason for it. For example, he states that making a character blind must have literary significance because it is much harder to make a plot when the character has to remain blind for the entire book. Shakespeare takes the risk of involving spirits in his play, even though they are difficult to stage. Sometimes spirits are not able to be on stage because the spirit does not have a body. According to Foster's logic, there must be an important reason to use them. However, since Ariel is a spirit, she has magical powers.  So while it may be difficult for the stage director to represent Ariel's character, in the printed version of the play, the use of spirits gives the author tremendous freedom due to the unlimited capabilities of magic.