Well, even if it's a relationship, it's still not a very good one. Where as relationships can be used to add something the characters, it's just a slight detail that doesn't really add to nor serve the story line in any way.She isn't a "love interest to Spock" -- she and Spock are in a relationship, which people tend to have in real life.
I disagree with your apparent view that their relationship doesn't add significantly to the film, probably because you seem to have a narrower definition of what the "story line" to the film is (i.e, defeating Nero). The relationship contributes much needed depth to their characters within the two hours without taking up more than a few minutes of the film.
Plus, what good is Uhura in a relationship if she won't stand up to Kirk when he's bad mouthing Spock on the bridge in front of everyone? Some relationship that is.
Perhaps because she is professional enough as a lowly lieutenant to not involve herself in an argument between the acting captain and first officer.