To lay out the dark-arts debate a little, we can discuss:

1. The underlying reasons why we dislike dark arts.
2. Whether we should like dark arts. 2a. - Efficiency (essentially assuming utilitarianism) 2b. - Morality (essentially denying utilitarianism)
3. Is there an "acceptable level of usage" (applies separately to 1, 2a, and 2b).

So you're asking about 2b and 3.

I'll make another comment to dive into that specific path.