Snarky Username said:
It felt that Shakespeare would just use the witches as his own personal Deus Ex Machina, just as catalysts for the plot to move it along or explain why something happened. Not to mention that many of the scenes would just be blown through, swiftly going from scene to scene and event to event.
The reason for all that is that Shakespeare wrote
Macbeth for King James I, who a.) had a
huge thing about witches, and b.) had a notoriously short attention span (it's Shakespeare's shortest play, and the only one with no subplots). There are also a couple of thinly-veiled references to the Gunpowder Plot that had happened only a year or so before, but that's a whole other subject.