Simple, Spock is the only familiar face from old Trek participating, they had to give fans something recognizable except for the picture.
Not to mention Nimoy delivered the lines before as we know, and to my knowledge so far, his role in the movie is the most important.
It seems like this might be an event were he'd be the storyteller for the movie(not exactly the narrator since he actively participates)and the axis around which it revolves.
Let's not make it a Shatner vs Nimoy or Kirk vs Spock thing and spoil it before it begins. I think a lot of the magic in the movies is lost before they're actually seen cause as fans we tend to overspeculate on every little tidbit.
always wears me out when I see that.