Excellent point! Interrogating the human (from the humanist perspective), and the presupposed qualities that make it up and then placing that into larger contexts about life on Earth is fascinating to me. And this is just one route that posthumanism is interested in.
This can be done in one sentence: Posthumanism is important because it forces humans to rethink the ways that they address the environment, animals, computers, and other f***ing people.
For example, N. Catherine Hayles draws from several science fiction authors to illustrate how silly the concept of downloading one's consciousness into a computer, an idea which many people think is totally possible and/or a good idea! Totally fascinating to me.
But I'll probably end up without a place in academia, according to many on this thread. Oh well, I'll try and find somewhere that'll let me teach.