PLD will always be a viable tank, its your standard hardcore mitigation meatshield and it's a good/safe choice whereas others may wain from patch to patch. Being that DRK and WAR serve a similar position in fights, it's likely that one will be preferred over the other for a while, and from time to time the preferred DPS-tank will vary based on changes to the jobs or to the game. SE is always trying to make all its jobs viable.

IMO, if you want to be a tank and you really like DRK, then level/gear DRK as your main, but get a second tank up there as well (either other tank will serve your purpose). This'll allow you to play DRK when a DRK is needed, and to still be tanking when DRK isn't sought after. Like you said, they share gear anyways. My current intention is to main PLD which is my favourite job, and get DRK up there because its another fun job and sometimes a tank with more DPS or a separate set of skills will be preferred.