This is all true. Also, a term that means "non-trans" is useful when we need to distinguish something about trans and non-trans lives. (That is just what distinctions are. We label distinctions so we can better see them and point them out to others.) It's not as though people go around labeling each other "cis" all the time for no reason, like "that cis person down the street is catching the bus."

Similarly, a term like "person with a uterus" can be useful when we are literally talking about people with uteruses — at a repro clinic, for example, likely one that mostly serves cis women but where trans men and nonbinary people are also acknowledged and welcomed. "Person with a uterus" isn't used as a general substitute for "woman" as in "that person with a uterus is catching the bus." It would be inappropriate and unnecessary, and no one does that or is arguing for that.

We use the word "cis" when we have reason to specify "not trans." Sometimes a person's cis-ness is known and relevant to whatever is being discussed, just as a person's trans-ness could be known and relevant. Or: cisness and transness can be discussed in the abstract.

"Cis" makes transphobes' heads explode because they do not consider trans men and trans women to be "real" men and women, and thus they do not like the implication that a person can be a man or a woman in a trans way. They may acknowledge othergenders like "transman" and "transwoman" but not "men and women who might happen to be trans, which is equally valid as happening to be cis." They think the only way to have a real gender is the cis way, so to them the word "cis" is redundant, and they use the label "trans" to flag someone's gender as fake.