I don’t know if you had a chance to look at the VFX Featurettes from Oblivion which shows the use of projections instead of a green screen replaced by matte painting of plates, but it’s really interesting.
It’s interesting because it seems to be the future of green screen replacement. At least, this is what people say in this featurette. And that’s true, it’s really helping a lot the shot. All the compositors know that it’s a nightmare to make a full greenscreen shot working with a plate. Light never really works on both foreground and background, there is a lot of despill to do, and you keep this layering feeling.
That’s what happened when Lucas shot the new Star Wars trilogy with an empty set. The result is not always very nice, just because it’s impossible to replace big blue or green panels in a realistic way, during a long sequence.
So yes, it’s absolutely right, being able to use projection of a plate which both fill the background, and give the correct light/reflection on the sets and on the characters is a bright idea !
They probably haven’t’ been able to keep the BG as it is though, and I’m sure they had to replace it later with another one. But at least, the light on set was right, and even if they had to roto the actor, unless you have hairs or very small objects moving in front of the BG, it’s not dramatic. We would have to roto a lot of things out anyway, even if we had used green/bluescreen. In a matte painting point of view, it’s really useful to have a reference of what give the light of the set. Even if the resolution of the projection is not enough, you clearly see where the light is, the values of the colours, intensity of lights. It makes your work much easier when you want to repaint something then.
Have a look at this video, it’s funny to see how front and rear projections, this amazing techniques we used decades ago are still up to date, even if it’s just to provide a nice lighting and reflection pass, before we put a better BG in place.