I'm A Banana
Potassium-Rich
When designing, the first thing I do is look at the rough level range of the adventure I'm creating. Then I make a list of the creatures that fall into that level range. Then I pick the monsters that either fit the theme I'm incorporating into the adventure, or monsters I can respec fairly easily to fit into the theme of the adventure.
For me, the DDI is for this way -- the way you usually do it. A searchable database. And I'd love to have more in there than "things to beat up." I'd love to see encounters keyed to critters that involved things other than raw combat, too. I want to see the skill challenge involved in uncovering the wererat's true identity laid out for me along with the wererat's combat stats, ideally.
I can't think of a time I've every worked in the other direction - Hrm, here's monster X, what interesting adventure can I make from what I have here? It's just never occured to me to work that way.
For me, this is how I use a monster book. A compendium of plot ideas. And I'd love to have more in there than "things to beat up." I want to see mini-adventures and hooks dangling from every angle and story information that compels me to include the monster in a future adventure.
From my perspective, the idea of including things that aren't there to kill is essential both to my direct and indirect prep time, as a DM.