My campaign settings tend to be pretty "monster" light - plenty of humanoid baddies, but few magical beasts, abberations, outsiders, and so on - so no, I don't really need a lot of monster books.
I like templates - each template can take an ordinary goblin and turn it into any of a number of different kinds of nastiness. Templates are easy to write, and a few go a long way, so again, I don't need books full of them.
With the ability to add class levels to humanoids and many critters, again, it's easy to create a unique challenge from an otherwise "mundane monster."
I tend to create my world first, including the races and species that populate it, and let my stories arise out of the interaction that results, so in that sense, the monster comes first. However, the stories tend to dictate what critters are involved, so it's kind of a chicken-and-egg thing for me.