There are bad tanks and there bad DPS and bad healers, but it's more likely to succeed if everyone follows what the bad tank believes than what the bad DPS believes, so you might as well go with the tank, even if he's wrong. There's no dungeon that's so hard where even if you're marking the '1' in the worst possible way, you'd somehow wipe by attacking the '1' first. Honestly the tank really doesn't affect the group that much unless he's purposely pointing frontal AEs at the group, or standing on instant kill ground effects on purpose. So while most tanks are indeed bad and have no idea what they're doing, there's no reason to argue with them if you know what you're doing because the only thing you can't overcome is if the tank dies on purpose.