The only way the exhibition "limited play book" makes sense is if they have an entirely new play book.
Otherwise won't they be running, for the most part, plays they ran last year? Plays which any opponent will have seen on film? And once you spring these "mystery plays" you've been hiding until the first game, does that mean you'll not be effective the rest of the season because - Gasp- you've shown your cards?
I think it's all a bunch of psychological hooey.
Disclaimer: I have not played football since Little League, but I did stay at HI Express last night.