Basically adding to what everyone else above has said..
MM is almost defined for it's sidequests and collecting of masks which you can do through out the game. Those masks can be more puzzling to acquire than actually completing the game, some are near impossible to get without the help of a Walkthrough, (or atleast I needed it for one or two.) FDM is the reward for completing the selection of sidequests which you encounter in the game, and even then the small mini-games that the demented children tell you to play in order to acquire said almighty mask, can be quite challenging.
Even after all this, if you do not think the Fierce Deity Mask is a fair prize for the effort you have put into the game, then you don't have to use it?