Let us consider, further, the "meeting of the minds" theory of translation.* As I suggested in a post below, a good or great poet translating another good or great poet might lead to a happy synthesis of poetic talent, or a meeting of the poetic minds. So we might welcome a translation by Celan on Dickinson because we are interested in Dickinson, or Celan, or in the meeting between them.
This seems plausible on its face. One idea would be that the poet-translator could produce a remarkable poem in the target language, using his own poetic talent. A second, that the poet-translator has some special insight into the poetic art of Emily herself, and can "translate" that understanding into the German poem he is writing.
The second theory, then, is based on a kind of Bloomian notion that poets have special insight into other poets, even (or especially) when this insight is based on error. We can see this in some literary criticism by literary geniuses, like Beckett on Proust. Beckett's insight is extraordinary, but it might end up telling us more about Beckett than about Proust himself.
Another parallel might the idea that we might want to hear a great performer playing the music of a great composer. We get great music twice over, and we get to also consider what particular aspects of the great music come out in that particular performance, with that added intellectual pleasure of the analysis. This is a richer experience than simply reading the score on the page or listening to an indifferent performance to remind ourselves of how the music goes. This analogy is inexact.
To translate poetry, one must be a poet. In the first place, one able to produce the poetic utterance in the target language. Translation is a decoding, yes, but it is also a re-encoding. Secondly, there must be a meeting of the minds, a relation between two poetics (assuming the two poets never share the exact same poetics).
Defenders of translations undermine translation by being less exigent with the second poem, the translation. People will say, well, "that's difficult to translate so give him / her a break." My position ends up being extreme but based on very sensible reasons as well. In other words, it is perfectly reasonable to say that translations should be good poems in the target language, and almost everyone agrees with that. But if you actually have that expectation with real translations you will come off sounding extreme and intransigeant.
*That came out sounding stuffy. Sorry! My prose is off-kilter today.