Okay. Let's see. I think it would sound something like this:
"Hello. I'm alien and I would like for you to prove that humanity is worth saving."
"How do you expect me to do that? I'm not aware of any way of scientifically prove the value of a life form or society. I think it all comes down to how you determine the actions worth. Are you thinking morally or economically? Humanity is a very mixed lot, and if what you want me to defend is the core human nature then it's really hard because we ourselves do not fully understand it. One thing I will say though is that humanity is very capable of self awareness compared to any other earthly specie. There is a lot of ignorant people, but it's in the very nature of humans to ask questions and seek out answers. Thus we learn and evolve. I'm probably not familiar with a fraction of the worlds species, but I have been told that humans ability to think that they think is something we haven't found in any other race. We don't just think something, we ask ourselves why we think that, and we have a drive to be 'correct'. Above all, we learn. Because you're here communicating with me I'm certain that you too have the drive to learn. And because you ask me why humanity would be worth saving, and I'm beginning to suspect that we have a lot in common. What I believe in and hope for is that humanity will keep learning about itself and improve. Not just scientifically. I think we will become even more self-aware and build a better future where we work together for the common good. If nothing else, we share a dream of a world where everyone can be happy, and we're not quitters. If you want something in return for saving us, what I can offer is analytic mind of humans. I'm sure there are great thinkers among you, but being of an entirely different species I'm also sure that we could provide a new way of thinking. Asking is the first step towards learning, and we ask a lot of questions."
I think I could make quite a compelling case.
Might make them look at this while I'm at it: http://www.youtube.com/watch?v=l7AWnfFRc7g&feature=channel_video_title
It was pretty convincing imo.