A strong motivation for a lot of people identifying as non-binary is that if the boundaries between different forms of gender identity and expression, as well as sexuality, are mostly or completely removed then the language for describing somebody specifically as transgender or as particular non-binaries would be unnecessary. Everybody would be allowed to pick and choose what traditionally masculine and feminine characteristics they identified with and performed, as well as who they had romantic or sexual relationships with. And if pressures and stigmas would be removed from our social subconscious, pretty much everybody would be seen much more ambiguously than the predetermined (authoritarian) groups we have now.
The argument goes that strong association with a series of traits associated as masculine or feminine is a reaction to growing up in a society with tight boundaries on the definitions of both of those groups - not as an inherent human characteristic. So in a society where male and female were more loose concepts than harsh bound groups people would never end up with the psychological attachment to either particular side.
Instead, an entirely non-binary people would have the opportunity to choose throughout their lives the exact traits and characteristics they liked and live by those, traits which they can pick and choose from freely without having to group them together in premade configurations. If a person wanted to pick a bunch if traits that, as we understand them, are feminine - so be it. But that would just be viewed as another one of millions of unique combinations which anybody is free to group however they see fit - not adherence to a strict social arrangement.