The little routed out area is only for screwless plates, as most of the Lutron devices end up being proud of the surface once the plate goes on when using this type of box. Plastic boxes end up sticking out more...
That was part of my problem in trying understanding this, as I have never actually seen a switchbox that was mounted using recesses like you are describing. Were it not for the fact that I happen to have an Insteon dimmer with precisely that problem (to wit, I tried to put a screwless plate on it, and it stood out from the wall, so I reverted to a standard screw-on plate), I probably never would have figured out what you were talking about.
