I'm not sure if it was just the author's imagining or part of a bit of cut material, but in the novelization, Uhura goes to the Vulcan embassy and Sarek, just ahead of Federation security and gets asylum, which is how she got to Vulcan.
Not sure what the actual reason for her lack of usage was, but I can easily imagine the execs considering that not as exciting so they wanted it left out.
Yeah, the novelization was top notch, full of awesome extra bits like this -- it was by far my favorite novelization. I remember reading it as a kid and getting at least 100 pages into it and still hadn't reached the first scene of the movie.
The novelizations of both II AND III had a lot in them that was cut from the final films. The whole Saavik/David subplot being the biggest chunk. Uhura's expanded part in the breakout of McCoy and the theft of the Enterprise
was the other one.(Not only did she provide covert transport to the ship, but she hacked Starfleet Communications and scrambled them up so badly that only the Excelsior even knew about the attempt for some time. THEN she made a run for the Vulcan embassy.)