Actually feature films is the slowest to adapt GI lighting. There is a “don’t rock the boat” feeling. You will see GI used nearly exclusively in archviz, and you are starting to see it more and more in broadcast stuff such as commercials. Ironically, in terms of features, more GI is used in full feature an animations like Shreck and The Incredibles.
One of the reasons that GI is not used to much in features is that the industry is in tied to renderman which is not geared towards GI lighting.
But to answer your other question. There is no reason why GI can’t be used. There are several tricks to make it work, including baking of lighting with point samples or textures, adaptive sampling, etc…
As far as compositing, a rendering engine like Vray breaks out the lighting nicely so that it can be recomposed:


