The way most encounters are designed, a tank's failures are the most highly visible.

Ravana, even in normal - if the tanks don't know what they're doing they will cleave the party and kill people, resulting in a wipe.

Almost any fight with adds, if a tank isn't prompt about picking up adds they will aggro a healer or dd and potentially kill them.

You can be a halfway decent dd or healer and just muddle through. It's a lot harder for a tank to muddle through anything but basic dungeon content.