Janine,
I think you are right that this kind of device was not used in any other Shakespeare play (if anyone can think of one, please let us know! ). Even though initially it seems puzzling and somewhat out of place, I enjoy reading the Inductions. They are rather funny and, in my opinion, goes well with some of play's themes (I will not go into this as I don't want to spoil it for those who have not read the play).
The change in Sly's attitude and his willingness to go along with the 'deception' is very interesting and can be true for all of us under different circumstances, can it not?