Because continuing this conversation in the "stupid things people say" thread was... well.. stupid.
My opinion on the situation is that pretty much everyone involved is a selfish jerk!
The heart of the issue is that people like labels, and like being able to label people, because people want other people to fit into neat little categories, while also wanting their specific category to be seen as the default. They want to be normal, and they want anyone who DIFFERS be those who have to be explicitly labeled. This isn't to say they don't want a label - "normal" is a label - they simply want to decide what that label means.
I think there's a purpose to a word like cisgendered, because it's a useful qualifier in the kind of conversations it normally comes up in, and defining things by what they are not ("not transgender") is often inaccurate and fails to accurately describe what is being discusssed. That's fine!
Of course, the term has since been thoroughly abused, and instead of being a positive declaration of attributes (not in the sense of positive morally, but rather being a label defined by qualities a thing possesses rather than those it lacks) it's become a simple synonym for untrans - like many elements of language in the social movement, it's devolved into filling the same tribal purpose those who use it purport to advocate against - it's become nothing more than a negation (often a slur and ad hominem), and lost, in general use, many of the aspects that made it especially useful as a term.
Of course, the original need is still there, so it's going to continue existing even if everyone stops being idiots, but because everyone, everywhere, is too caught up in their own goddamn ego and need to feel important by excluding and trivializing the opinion of others while trumpeting how it's important that exceptions be carved out for them, in particular, because every improper use of language that doesn't serve their personal goals and soothe their ego is obviously an attack on their identity... blah.
Saying "I'm gay/straight" is much easier than "I'm not attracted to your sexual morphology." Terms have a point and purpose.
And herein lies the problem. Terms have a point and a purpose, but "accuracy", "clarity", and "acceptance" are very rarely a consideration. "Convenience", "identification", and "exclusion" are far more common.
"cisgendered" (and "transgendered" for that matter) can certainly be used for the first three, just as "gay/straight" can be... but if one thinks that's how any of these terms usually get used, I have a strong suspicion you may be mistaken.
Essentially (finishing analogy)
When the boys set up their clubhouse and put up the sign "boys only, no girls allowed" (and, as is implied, no transgender or androgynous folk either), the natural inclination of those that remain is almost always "Well, fine, we'll set up OUR OWN clubhouse", which promptly puts up a sign saying "girls only, no boys allowed" (and, as is implied, no transgender or androgynous folk either), leaving a smaller group of excluded people who are now even MORE excluded. This is followed by the transfolk saying "Well, fine, we'll set up OUR OWN clubhouse" and promptly posting a sign saying "trans only, no cis allowed" (and, as is implied, no androgynous folk either).
And etc. and so on.
Turns out most of ANY group is full of bigots and assholes. Victims often become bullies themselves, and bullies are often victims, because it turns out what most people want isn't justice and equality, but an excuse to think of themselves as superior by excluding others.