I guess I just do not see the cost, you can give your vendor an image so it arrives at your door with the custom OS and configuration on it, GPO's aren't a maintenance issue, setup properly as a thick client the configurations are static and you can launch a published desktop exactly like a thin client, there is no functional difference. They can't install anything locally, they can't change settings, they just launch a desktop off the thick client and go.
What thin clients do for you that isn't in black and white is turn your help desk in to a cd reading factory for users because they can't anymore, also lose the ability to attach devices like scanners easily so you end up buying some desktops anyway. It's a measurable loss in versatility and productivity and forward movement because anything that does not fly with your proprietary hardware sinks the ship.