You also get people defending Cure I in 90 content, tanks that don't CD at all because "it's the healer's job to heal" and DPS who haven't discovered what AoE buttons do.
Some people are dumb. You ignore them. Anyone saying it's impossible or even remotely difficult to play Dark Knight in any content, much less dungeons, has never played any of the tanks before, including Warrior. Either that or they're lazy and want to AFK watching Netflix instead of occasionally looking at their screen to maybe toss an oGCD on a Dark Knight or Gunbreaker. That's why they want a Warrior.