I agree with naushika that Voldemort probably would have gotten another object important to Hogwarts instead. What, exactly, I'm not so sure, but I'm sure there's many things in the castle we don't know about. It would have been harder for Harry to find it, probably, but maybe Dumbledore would have been able to figure it out or checked up on things in the castle that Voldemort might want to take.

Chelsea//Ravenclaw