Potential Edition customers won't have to wait, they'll be put at the front of the queue and will be seen to by an Expert, not a Specialist- assuming the store carries it. Because not all locations will have a supply of them on hand, the appointment means they can have one delivered into store for you to try.
Also, potential customers won't simply be turned away if they haven't made an appointment. You can leave your mobile number and you will be sent a text as soon as a Specialist is available to see you.
The point of this system is to slow down customer interactions and thus allow potential customers to take their time in getting to grips with whatever Watch they have in mind and try the different bands. Apple Stores are just too busy to allow for the usual free-for-all regular product launches become.
Finally, product reservations are nothing new. Apple did it for the iPhone 6! If you know which one you want, cool. Reserve online, then pick up on the 24th at the time slot you've chosen. You have two weeks before release to get into store and have a play about with a Watch and decide.