A minor tidbit here that might or might not relate to the issue of Uhura deserting her chair:
Neither in TOS nor in this movie is it ever suggested that Uhura would have been the top-ranking communications chief of the vessel. In TOS, there were several Lieutenants holding that position, some played by actors older than Nichols. In the movie, Uhura wasn't even supposed to be aboard the ship in the first place - and the Enterprise
would have been the one ship with the best odds of having some "real" crew around, as she was actually expected to set sail sooner or later.
Ensign Chekov certainly wasn't the Navigation Chief in TOS, and quite possibly Lieutenant Sulu wasn't a department head either (except of course in the pilot, where he appeared to be the Chief Science Officer). It's just that Starfleet seems to prefer to have its personnel working in teams, so that certain people are grouped with certain other people during certain watches. Presumably Starfleet wouldn't want to cluster all its department chiefs on the same watch. And presumably Starfleet trusts its junior personnel and doesn't expect the chiefs to rush in to relieve them whenever the red alert klaxon goes off.
However, STXI had exceptional circumstances regarding the bridge crew arrangements (thus maintaining a proud ST movie tradition). For all we know, Uhura was warming an assistant comms operator's seat there, and was not required to maintain vigilant watch or anything. Heck, that she was on the bridge was just a bit of favoritism from Spock to begin with; perhaps she had only make-work?