We checked with John Carmack himself about why DOOM 3 will be hard-capped at 60fps in the renderer, and he had this to say:

"The game tic simulation, including player movement, runs at 60hz, so if it rendered any faster, it would just be rendering identical frames. A fixed tic rate removes issues like Quake 3 had, where some jumps could only be made at certain framerates. In Doom, the same player inputs will produce the same motions, no matter what the framerate is."