Email idea is okay. But keep in mind, Passbook IS able to function in a lot of great ways, but just isn't implemented by most developers. For example, American Airlines COULD ask for your email, and mail you a link that creates the Passbook ticket.
Anyone can make a Passbook ticket of any kind right from a URL: http://www.passsource.com/
It's up to developers to implement this in better ways (as opposed to having to install an app)