Depends on how and where the master was created and what source files were used. If the source files, after color correction were ProRes 422, and mastered in Final Cut then there may have been a gamma shift to 1.8. If the cc'd sources were ProRes 4444 then the gamma 2.2 would be preserved throughout. FCP changes the gamma from any source, from 2.2 to 1.8 (Apple determined viewing) except ProRes 4444, which maintains the SMPTE standard 2.2 gamma if the source was graded that way.

So to answer your question there is more usefulness for ProRes 4444 than just bit depth. As a colorist I only render to and deliver ProRes 4444. Unless rendering DPX or Tiff.