Trans people are hypocrites because they impose their own schema of what gender is onto people with a normative, non-trans schema of what gender is, and then label proponents of the latter as cissexist, discriminatory, etc. In reality, both trans and normative understandings of gender are valid.
I've dated a couple of transgendered men and women, and this is absolutely true. I've wasted many long nights trying to explain that their definition is just as valid but not more valid than hetero-normative men and women. It never ceases to amaze me how many of my "liberal", left-wing, alternative lifestyle friends are so completely and irrevocably narrow-minded.
"Cis" came into use because when people tried to talk about trans* verses non-trans* people tended to use things like "normal people" which is hurtful and implies trans* people are abnormal, inherently, rather than it being apart of their identity. It's just nomenclature to make discussions more open, and easier.