Comet Siding Spring has a chance to hit Mars.
Why should we attempt to save Mars? This is an opportunity to prove our best method for diverting such an object if it were headed to earth. In addition, if the comet does hit Mars there is no way to know if the resulting impact could launch debris toward Earth.
In addition, this mission could be just one of an entire suite with the express purpose of testing and proving methods of preventing disasters from space. Since this mission is not to save Earth, more than one method can be tried.
If we ever did have such a challenge directed at us, how much better would the population react if they knew we had already tested and proven methods to save ourselves? If you think about it, that's what we would really gain, saved lives from not panicking.