I really want to state that the MM is not core. You could purchase any other book-o-monsters and get the same thing. Am I the only one trying to convince WotC that they should re-publish the 1st and 2nd monster manuals to contain an official CR/ECL system, and a precise way to generate monsters (step by step, like the process for creating characters in the Player's handbook).
The 2nd book would contain new creature feats, new special abilities, even new TYPES of creatures. The 1st would define the mechanics.