Absolutely. We would be backing up the biosphere. We wouldn't just be preserving humanity, we would be preserving much of life. It is certainly possible for some calamity to come along — as we see in the several major extinction events in the fossil record. Humanity has obviously developed the means of destroying itself, so I think we need planetary redundancy to protect against the unlikely possibility of natural or man-made Armageddon.
It is important that we take action now to make life multi-planetary, because this is really the first point in the 4-billion-year history of Earth that it has been possible. That window of possibility will hopefully be open for a long time, but it may only be open for a short time. That's why I think urgent action is required on making life multi-planetary.