The secrecy is definitely better for "buzz".
Right now, everything is speculation, and every bit of those mounds of speculation is something we can all argue/discuss/debate. As it is now, the sky is the limit when it comes to possible plots and character MO's.
HOWEVER, once the plot is revealed, those mounds of speculation go away, and all we are left with is the actual plot and the actual character motivations to nitpick (and ST fans are sure to nitpick the plot).
So from the movie studio's standpoint, the secrecy is good because it creates greater amounts of speculative buzz and a whole lot less nitpicking.