It takes a certain amount of discernment to, well, discern allegory from history.
I admit it is much easier to simply read the entire Bible as history, but was it written to be such?
But the danger is that when you say (as some on this board have said) that all or most of the Old Testament is metaphor, you run the risk of having the same metric applied to the New.
A good example is the case of Onan being used as a proof text against contraception. Was that allegory?
Look at what happened to the ELCA. They started down the path of historical criticism and allegory, and ended up with sodomy and apostasy.