Apart from brake cooling, there is also a MEL that permits 747s to takeoff with one or two brakes deactivated provided that the gear is left down for two minutes.
In my part of the world, its very common to see aircraft leaving the gear down after takeoff due to cooling purposes. You have to remember that all takeoff calculations are based upon an engine failure, so in the case of brake cooling you have an un-failed engine to account for the increased drag from the gear.
Mutt