Why is it important that it is Simon who discovers the truth about the beast: that it was harmless, but horrible?
Simon serves as a religious figure in the novel. Therefore, he often uncovers amoral behavior and criticizes it through either internal dialogue or with other characters. The beast was harmless because it did not exist. However, Jack was able to make the threat of the beast real and thus enforce his ruling policies based on a fake threat. We see some of this behavior with leaders in power today. Citing a threat to the general public and leading them to beleive a threat is real allows for public approval of policies that remedy the threat.
Also consider Simon's death. The boys believed that Simon was the beast just before killing him. The boys' first instinct was to kill instead of find out if the "image" moving in the darkness was actually a beast and not another person.
Moving to kill first and ask questions later was a profound symbol of the boys' decent to savagery. In addition, it is said that a lot can be told of a society in the way a death is mourned and a funeral respected. The boys, however, simply allow Simon's body to float out to sea.
It is quite a while since I read the novel - 40 years and rising - but I do remember that Simon came across to me as a symbol of all that is 'good' about humanity, that is, he is generous, kind, well disposed to others, but also interested and enquiring and courageous in his way. In other words he is the opposite of the superstition- and fear-ridden savagery that takes hold among the majority of the boys as soon as the constraints of 'normal' society are removed. It might not be over-stating it to say that he represents enlightenment, maybe even The Enlightenment, and it is therefore appropriate that he uncovers the reality of the beast.