I don't think there's anything that we could learn from gain-of-function research on deadly viruses and bacteria that would be worth risking millions of deaths.
It doesn't matter how good your intentions are when your behavior is extremely dangerous and can (and possibly did) result in a global pandemic that kills millions of people. Risk/reward calculations should be performed without regard to intent.
>Banning it would reduce the amount done greatly.
It would also reduce the risk of a man-made virus killing millions of people.