It's kind of a chicken and egg scenario.
Good tank/bad healer: Healer lets tank die, chaos follows, likely wipe.
Good healer/bad tank: Tank doesn't hold aggro over healer, healer dies, likely wipe.
Good tank/bad healer: Healer lets tank die, chaos follows, likely wipe.
Good healer/bad tank: Tank doesn't hold aggro over healer, healer dies, likely wipe.