I'm not sure the MM is about "story needs." It's your starter kit, so to speak. The monters in the MM3, to my view, show the same philosophy, and differ only somewhat in the proportion of various types of monster. As the review notes, the MM3 takes pains to cover uncovered territory. That's definitely a feature, though. One thing D&D does not need is more redundant monsters. While there are some trivial inclusions, for the most part, the MM3 focuses on fairly original beasties rather than old standbys (like the MM) or quirky surprises and obligatory throwbacks (MM2 and the Fiend Folio). If you really want more demons, devils, and dragons, take a look at the Draconomicon and the Fiend Folio.

In some ways the MM3 foreshadows the plethora of beasties of various types and characteristics to be found in Frostburn, Sandstorm, and so forth. And it avoids obligating Realms or Eberron fans to purchase the MM3 for completeness.

The MM3: a supplement that actually supplements, rather than obsolesces or supercedes.