Alien civilizations don't have to exist for the Drake equation to be correct. Zero and one are perfectly good solutions.
Maybe I am getting "correct" and "useful" confused here. The equation can suppose all the intelligent, communicating civilizations it wants, but all its supposition is unproven.
I think we're getting into the realm of semantics here. Bring me proof of ET and I'll have a lot more respect for the Drake Equation.