Drain = Hits * 2 is carry-over language from 4E and 5E.
I believe the general idea is that banishing is potentially "easier" than combat because the spirit's number of services basically act like a separate, shorter condition monitor. One hit on a banishing test might be all you needed to get rid of the Giant Spirit of Doom - indeed, the larger the spirit, the fewer the services it probably had, and so the shorter its damage track - so the drain to the banisher was a way to counteract that.
But in all my time playing Shadowrun, I think I've only seen Banishing used once, because 1) nobody wanted to make the investment in skill points, and 2) nobody wanted to risk the drain.
6E's approach to skills (which, to be transparent, I love) means that #1 isn't an issue anymore since you use Conjuring for both summoning and banishing. Now your only deterrent is the potential drain.
All that said, if I were in charge I would probably make the Drain = Hits because I would prefer the symmetry with summoning. I agree that if summoning an F4 spirit isn't a fatal enterprise then banishing one shouldn't be either.