I think basically the common movie goer wants to see films that are fun and entertaining. And it just so happens that the trend in fun and entertaining films being made today are the sci-fi/fantasy/superhero films. I don't think movie goers are "huge sci-fi/fantasy fans" in common sense of the term, but instead they just like to see fun films.
That's why I do NOT
think any harm is being done to this film by Abrams secrecy. The vast majority of the fans who actually care about the secrecy will see it anyway -- secrecy or no secrecy. The rest of the movie-goers (the general population of average movie fans) don't care that much about movie hype 6 months out anyway. They only say to themselves and their friends "what looks like a fun movie that we can see this week".
...And sci-fi/fantasy/superhero films usually look to be the most fun to the average movie-goer (sci-fi fan or not).