I tank on the DK because that 4th ret is too valuable.... not because the DK is a superior tank. Also - switching a pally to prot means the DK goes dps... which he fails at.

The extra dmg and healing from a 4th ret more than outweigh the extra dmg and PITA factor of tanking on a DK. Plus in PVP I prefer having my DK as a huge dmg sink... so full tanking gear is a plus for this also.