To allegorize is to speak on a subject by speaking about something else. There are thus two problems in an allegory. The first is that the hearer will not understand that one is really speaking of something else. The second is that the hearer will not perceive a connection between the actual subject and the something else. Aesop probably had more problems with people misunderstanding his fables than with people believing he actually had talked to these animals and heard the frogs tell of when they demanded a king or saw the grasshopper, fiddle in hand, begging sustenance from an ant.
On the other hand, Biblical allegories are for the most part understood to be allegories, but they often are also actual events. The reader is likely to place the Bible's allegories into the same category as Aesop's and deny the historicity of the Biblical event as much as he denies the historicity of a group of mice trying to bell a cat.
Doing this, however, is as foolish as denying that Isaac and Ishmael were ever born to a man named Abraham (despite the fact that all Jews and all Moslems attest that this happened) simply because Paul uses the two lads as an allegory in Galatians 4:21-31. Denying the historicity of the allegory is really denying that an omnipotent and omniscient God superintended the events in the Old Testament, which the Old Testament clearly claims is what happened. Exodus 7:3 states specifically that God hardened Pharaoh's heart so that His signs and wonders would be multiplied in Egypt. This is a hard saying, but one may also recognize that Pharaoh at any time could have simply let God's people go; he never lost his personal ability to choose. Generally, one will deny God's omnipotence because to acknowledge his ability to control events in the past is to admit he can also control events now, including one's own life, which is a frightening thought indeed. So the first rule of the Biblical allegories is that they exist: Go...