There was a good discussion on takeoff roll and spooling up some time ago. YouŽll find it at:
http://www.pprune.org/forums/showthr...s&pagenumber=1
The actual timing of brake release in the whole process of spool up, stabilise, then full/flex power is quite important from a jet blast design process. Many runway and jet blast clearance designs assume that there is about 100m between the end of the runway and the achievement of full take-off power. IŽve agonised over this a couple of times where clearances were tight. It can make quite a difference in the design blast velocity.
It is not quite as haphazard as it seems, because there is some statistical element of probability included in this. If a runway end is close to the fence with a public area of road beyond, then if only 1 in 10 aircraft winds up while standing on the brakes, then the risk of a person/vehicle passing behind during start of takeoff run is reduced by one order of magnitude.