Hm. I'd figure any alien race with technology advanced enough to get here (probably through space/time manipulation; 4th dimensional shit rather than an advanced grasp on physics in the 3rd dimensions..) would probably also be quite peaceful. Why? Well, if they've managed to survive long enough to develop such advanced technology, it makes sense to me that at some point along the line they may have gained some form of species-wide enlightenment that gives them respect for all other beings of their race, and with enlightenment and a respect for your own species, logically enough, comes a respect and understanding of the equality of other species, too.
If they were to still pop the question and ask why we're worth saving I'd simply point out that both us and them have the gift of consciousness and that given time and a little shove, we could evolve ours to a similar point as their's. Surely us sentient beings should stick together, given the potential for our awareness can only increase provided we don't wipe ourselves out?
But given what I said about my theory that such a race would be quite enlightened, I am pretty sure they would already have realised that all things are a part of the divine oneness and that there is no reason for violence towards us.
Unless we're talking Reptilian Aliens such as the ones David Icke talks about.. In that case, short of finding a way to make them feel empathy en masse, I'd say we're pretty fucked.