I agree with what you're stating and I hope that that's where we are headed - there just being man, woman, and non-binary gender identities. For now though, as I see it, cis/trans become relevant in three ways (off the top of my head):
- Medical situations - self explanatory
- In pursuance of romantic/sexual relations - society is still at the stage where the visual perception of a person lead to certain presumptions. Not everyone may be okay with dating/having sex with a person who has different genitalia than they presumed that their partner would have.
- [This point is my personal opinion] The visibility of people openly owning their journey can be a source of information, give courage to others who are undergoing the same journey and are scared or confused. I knew about my orientation way before I realized that there's a term for it or that their are others like me.
I hope I got your query right.
Thanks for the cheat sheet idea! That might help!!