They could if they wanted to I suppose, but they won't. Because that would mean balancing each tank class for 2 seperate roles, and they'd have to have different skills, since like 1/4th of a tank's kit is defensive skills that DPS classes don't have or need. And if they didn't do any balancing like that, tank classes in DPS roles would be horribly sub-optimal since they naturally do less damage than DPSs.