That's the question asked recently on the LinkedIn Conference Planners answers page, and they got some interesting answers back. Like this one:
Believe it or not, one of the best ways to get the best possible price on airline tickets is by using a meeting planner. If the meeting planner is big enough, they will be very well known to the airlines through consistent and prolonged purchasing of large quantities of tickets. The airlines are motivated to provide these group the best possible fares that most individuals cannot obtain on their own. If the event is large enough, the reduced airfare (and usually reduced hotel costs as well) will more than make up for the fee charged by the meeting planner.