I think the issue of complexity, especially as it relates to prep time, has more to do with use than anything else. No one (okay, I am sure sameone does) wants to spend two hours statting a creature/enemy that is going to be good for one fight. Give me powerful, flexible opponents with simple stat blocks and limited tactical choices, please!
However, lets not throw the baby out with the bathwater. If the use is more long term -- recurring villain, behind the scenes manipulatror, that sort of thing -- the options must be there to do more than have insteresting tactical decisions to make. Demons and dragons make excellent long term villains just for this reason -- they have lots of options which means that when you need them to be able to so something non-combat oriented against the party, they can.
If creature design goes the way of "what combat role does this monster fill" then you end up in a situation where you basically have to use NPCs with classes, etc... as villains all the time, or create things from whole cloth and hope they are balanced. Isn't it better to have as many options as possible, combat oriented and not, in the MM? (I am looking at you, wooden warshaped dryad.)