I don't like the cliche catch-phrase either, but I'd like to share what Joseph Harrington posted on facebook today:
". . . the poetic word is the one that is always situated in the position of a remnant and that can, therefore, bear witness. Poets - witnesses - found language as what remains, as what actually survives the possibility, or impossibility of speaking."
Giorgio Agamben - Remnants of Auschwitz . . . riffing on Holderlin's remark "Was bleibt, stiften die Dicther" - "What remains is what the poets found"
I generally dislike the term "poetry of witness," because in practice it provides a warrant for some rather cheesy, self-righteous posturing. But the way that he uses it here makes sense - witnessing not as authority but as a kind of abjection.