^^^ in addition to that, they can order a stop loss. a stop loss prevents a unit from losing troops prior to a deployment.
chris, i guess you can call that a dirty little secret, but its really a logistical issue. when a troop is assigned to a unit, they are taken into account as part of the manpower, availability, and readiness of the unit. these factors (and many more) are what help decide if a unit is ready for deployment. it would be a waste of money and resources to pull troops who ETS out of a combat zone, and then replace them with somebody else who the other troops do not know, and additionally did not train with said unit. further, it would be unwise to account for each troop prior to considering which unit to deploy because every unit is going to have soldiers coming and going. thus, the search would go on forever.
i hesitated in saying this many times, but it is because of bill clinton's administration that our armed forces are smaller. in order to get all the money his admin did, he severely cut the military budget which causes numerous base closing, reductions in troop levels and standing divisions. on a more personal level, senior enlisted (maybe those doggone officers too) who planned to retire from the military were often cut before they could retire which left them with basically nothing. they would have to start new careers in mid life with only military and sometimes combat experience. aside from the direct benefit for finance, it cut thousands of service members who would have to be paid retirement, etc.