This was posted by Joe Defuria over at beyond3d. I think it pretty much sums up why this is a cheat and not an optimization even though image quality stays the same so I'll just paste that quote here:
If the basis of your optimization requires you to have access to data that is NOT PASSED by the game engine in real time, then that optimization is a cheat. This 3DMark cheat is based on the fact that the drivers "are told" the camera path won't change from some determined path. Problem is, they are not told this by the game engine. Clipping planes are inserted based on this knowledge. That data (the clipping planes) are not passed from the engine in real-time, nor are those planes calculated in real-time (as evidenced by the lack of correct rendering when "off the rail".)
That is why this particular example is a cheat, and not a legal optimization. It relies on data that is not given by the benchmark, or calculated in real-time from data given by the benchmark.
This is why something like a "deferred renderer" is NOT cheating. It's not drawing "everything" either. But it calculates, on the fly, frame by frame, what is needed to be drawn. If you took a deferred renderer "off the rail" it would not suffer the clipping issues.
I couldn't sum it up any better, so I won't even try.