But when you speak in terms of speed in development I would have to argue against that.
My argument is 7 posts above. Give the level a spin and you will see that the technique does not work well for everyone. I'm not sure how long it took them to put "urban warfare" together but it would have taken me 30 minutes to do that level in worldcraft and 1 hour in radiant at most (a year ago and with a redbull). Certainly they could have at least made it at a playable state in a similar amount of time if your augment holds true. Actually it does hold true, but the skill needed is far beyond the vast majority of our development population and requires the years of experience few of us have had. I am speaking in terms of a single digit percent.
If what you say is true then how come the over-licensed Unreal engine 3 boasts unreal ed? It works alot like a modeling program (i admit & enjoy) but its still brushes. Im not saying thats the right way to do it at all - every licensee says it is. Radiant is still heavily used, Crytek has their own special editor, Valve has hammer. Gosh darnit we are pretty much covering the market aren't we?
As you said, not everyone can make their own special sauce or use someone else's. Sucks to be them. Guess they need to be good with the editor or be able to use it more efficiently with plugins or layouts. You can obtain better results if you model 100% of it but as we can see using real world examples, its not practical except for terrain.