Emergency medicine in the US is in a horrible state. Overcrowding is one of the primary reasons. See: http://www.ama-assn.org/amednews/2009/01/19/prsb0119.htm
One of the primary reasons for the large influx of patients is the diversion of primary care visits into the ER for non-urgent matters. A booking system that encourages non-urgent care visits to the ER as opposed to other urgent care options does nothing to help this.
As other commenters have mentioned, the problem with overcrowded ERs from a patient perspective where they have a potentially emergent case is NOT in the middle of the night - it's during the day, where other options (including primary care and urgent care facilities) are available.
Emergency medicine should not be treated in the same convenience form-factor as other consumable goods, and applying this approach is socially irresponsible in the big picture.