You're right, Predators usually won't erradicate an "infestation". They are most effective as a control measure when insecticides treatments aren't a reasonable option.(such as in flower) But via online search, I was unable to find a single retail outlet offering these beneficials in packets of "15-30". In fact, found none offering less than 100 and most were in the 1000-2500 range.Predator bugs usually come in packets of 15--30 bugs. If put into a growroom with thousands of mites, they'll never make a dent before the mites have ruined the plants, even though they reproduce twice as fast.
I've used beneficial predators before. The instructions say "knock down any infestation BEFORE introducing beneficials".