I'm about to post some concept sketches for my project, but before that let me point out this video. It's a small physics test that user "Elude" made to demonstrate a 25,000 barrel stack collapsing on itself.
I couldn't think of a better way to illustrate the benefits of a game engine over a 3d production tool. Running a physics simulation for that many objects at the same time would bring Maya to a screaming halt on the best of days. And splitting it up into smaller bits using the same program would be a time-consuming process. Using the engine's phsyics and rendering capabilities, not only is this feasible, but the creator is able to interactively control his camera as well.
He gives a brief summary of his method here: [epic games blog] but the short of it is that he recorded his computer's output using a program called FRAPS to capture the video as the game engine crunches away. All in all, it took about two and a half seconds to render a full frame. I have very strong doubts that this could be accomplished with both simulation and rendering in that time frame using maya/mental ray.
Score another point for hardware rendering!