The moral of the story is that the story is all that matters.

Act five to me is an epilogue. It's a chance to catch up with characters you had forgotten about until that point because they vanished from the plot. Like the extra scenes that appear during the credits of a movie, the end of MSND continues the story of the mechanics where it left off, the preparations for the play.  This play within a play acted out by Bottom and his merry men acts as a foil with which the audience can reflect upon the inverse story of the four lovers and the role that magical interference and coincidence play in the making of their happy ending. 

But why go to all of that trouble to tie up loose ends when you could simply have someone from Theseus and Hippolyta's court come out and tell you that they all lived happily ever after?I think that Shakespeare saw this as a chance to address his audience (us) directly; especially with Puck's final monologue.  The moral of the story is that the story is all that matters.  If you take away all of the myth, the fairies, the magical love potions, the Athenian Law, you are left with the same drama that saturates modern reality television.  It is the obsession with Myth that makes Shakespeare's treatment of the story  so that it needs to be seen both in the form of MSND AND Pyramus and Thisbe so that we can understand that it is all the same story. It is all part of the same myth, it has just been given different details. 

And like Puck says, if you didn't like it, just pretend it is a dream, because chances are you will see the same story again in a different form.