Well, I think the detractors would simply say that they don't think Tank is "good depth". Certainly BTS, the head of the naysayers, would call him bad depth because of his lack of present baseball skill.
In order for Viciedo to actually be "good depth" he would probably have to improve as a baseball player. He's been worth -0.1 fWAR in 1800 career PA, so the past and present version of Dayan getting playing time over someone even as mediocre as Pillar or Valencia could get pretty annoying.
And I don't think it's even fair to point to his career splits and say that in a platoon DH role he would be "good depth". See below:
Viciedo has just under 500 career PA against lefties.