It would be more like a couple of weeks or 1 month. I see what you mean here but I think that some customers may be interested in this approach because at this point they can offer suggestions for features and influence what is included in the app.

Showing them a mockup or signing them up for email updates at least would be something tangible so they have some piece of mind between the time of purchase and launch.

Tesla did this with the Model S, no? :)