I think allocation for reservations are only a percentage of their stock so it does sell out, the way I see it is it helps control the online crowd on release date and hopefully results in less lag/faster checkout?
While others feels good about being picked and it might become an impulse buy to some (if I were on the fence about buying because of the hassle of checking out on release date, if Nike allowed me to buy now then I'd do it on the spot), it might be a placebo effect to some but on a business standpoint, I see what they are trying to do.