As far as Thornton, what Jonathan means is that the whole team is executing a play run for Thornton. He's coming off of multiple screens and then getting the ball delivered by someone else and then having more screens set for him etc..
If you're only an okay scorer, that's an awful lot of work to do to get a shot for you. I agree that it's not a huge negative, but his point was more like this:
Thornton's not going to give you defense, rebounding, passing, etc.. He's going to give you scoring, except that he can't create his own shot or score in isolation, so you're going to have to have a lot of work done to get scoring out of him.
Thornton may be a guy like Eddie House. Eddie House can shoot off screens run around and shoot, sit in the corner and shoot etc, but he doesn't give you anything else other than that, and you have to work hard to get him the shots, and because he can't create on his own, it's not worth it working that hard relative to what he actually gives you.
Thornton can be a similar type of role player I think, and that has some value, but it's not huge value.
Terrence Williams has grown on me some, I was writing up my review for chicagonow.com on him, and I thought about it, and he's basically a guy who gives you everything but scoring, but as far as scoring goes, he can make catch and shoot plays, and will probably be effective eventually as a catch and shoot guy from the corner. So a guy who defends and can shoot open 3s. For a role player, that's basically exactly what you want out of the guy next to Rose.