Coming from FFXI and maining DRK and having gotten all the best gear for it through lvl99 cap, I am also on the side of wanting it to be a DD class. I know I am probably biased, but as the OP has listed, DRK has been a DD class in most iterations of its incarnation.

Yes they sometimes wear heavy armor, but that doesn't mean they are meant to tank. FFXI is a great example of that. DRK wore heavy armor and still was very squishy, but a MNK in light armor was able to take hits far better than a DRK could(until Apocalypse then DRK/SAM was both tank and DD haha).