I don't have the brain power for proper posts at the moment, but I can point to a short post on Alas, A Blog about why we need the term "cis":
Plus, as a political matter, it’s important that the unmarked “defaults” have names. Imagine if, instead of the words “Jewish” and “Christian,” we had only “Jewish” and “normal.” Or if, instead of “heterosexual” and “homosexual,” we had only “normal” and “homosexual.” We can’t discuss things on an equal basis without an equal vocabulary.