This argument proves too much:

"Bloodletting is not futile or make believe. It is useful even if the patient does not always survive. That's why physicians continue to use it."

"Trial by ordeal is not futile or make believe. It is useful even if sometimes Inquisitors reach mistaken conclusions. That's why Inquisitors continue to use it."

"Lottery-number-picking systems are not futile or make believe. They are useful even if some players never win. That's why players continue to use them."

It is a fully general argument which, if correct, would demonstrate that no practice that had continued for a period of time could ever be ineffective or counterproductive.