Let your exhibitors scan visiting users on their stands or use it as a seperate checkin to different areas to see how many that was attending the different sessions. Build competitions on which stand had most visits and which user visited most stands. You can even use it as a self checkin on a geo quest.
Waiting list option is used for an event with limited number of participants. Event organizer can let the event "open" with waiting list option in case he will have more slots available