On a human timescale, there is no sustainable harvest of [giant deep water corals]. We know next to nothing about how they spawn, settle and regenerate, but I have seen very few younger and smaller colonies, so even slow regeneration might not be a very likely option...-- Brendan Roark quoted here in reference to this paper.
Given their slow growth [however], we may be able to use them as high-resolution records of past climate change.
See also this 2008 note on an earlier study.
On old corals in shallow waters see Marine Methuselahs.