I suppose some people who request "X" do so because they consider themselves a "third gender." Certainly, many outsiders interpret the "X" that way. Though I tend to assume the "X" is a resistance to the gender question itself. The person who asks for "X" may really wish to burn the driver's license for asking such a question, but pragmatically they cannot burn it because they do need an ID card to function well in society, so they ask the system to fill an "X" in the field that most annoys them. It's an alternate form of protest. It means "X your terrible question." They may not mean to communicate that they literally, ontologically are an "X." (Whatever an "X" might really be, anyway.)
I'm just telling you how I tend to interpret it. This may be similar to or different from your interpretation.
Anyway, this perhaps mistaken/undesirable connotation—that the person with the "X" is a member of a third category—is a risk of keeping the field and of explicitly allowing people to fill it out with "X." In other words, as long as the "X" is seen as somehow invalid, it can be easily interpreted as a protest; but the more that nonbinary identity is accepted, it is because society increasingly understands it as a third thing, i.e., a third valid answer to the question of What is your gender?.
On the one hand, it is good and important for nonbinary people to be accepted as such. If they say "I like the word nonbinary and the letter X" then the word "nonbinary" and the letter "X" properly belong to them and others should use and respect those labels. If someone says they belong to a third, fourth, or fifth gender category, it is my responsibility to learn those categories and try to see the world from their perspective.
On the other hand, for anything to be accepted by the system means that it loses its inherent zing of protest to the system.
To make an analogy that refers back to my previous comment: Atheism is of course a valid philosophical position, but the more we accept the concept as such, we tend to forget that it's a protest of theism and that it wouldn't be its own thing (framed in this way) were it not protesting theism.
So, someone does need to explicitly make the argument: "But gender shouldn't be on driver's licenses at all, right?" And anyone can make that protest. The person who makes the anti-system argument need not be cis, trans, nonbinary, intersex. Not everyone feels strongly that being outside the gender binary is crucial to their personal sense of self, but anyone can make a political argument in favor of diminishing the power of the gender binary. I don't think it's fair for me to claim the label "nonbinary" because the gender binary doesn't aggravate my psychology as much as it aggravates some others (as long as I get to pick what side of the gender-fence I'm on, which I already did, many years ago, and I feel like one gender transition is plenty for this lifetime)—I mean, one side of the binary causes me dysphoria but the other side doesn't, so my dysphoria is within the binary and not about the binary—and I don't want people to refer to me with a gender-neutral pronoun mostly because I feel it just calls me out for being trans which is a wholly unnecessary thing to do to me. I'm not pro-binary, I just tolerate and cope with the binary better than some others are able to. That said, I do think it's everyone's political responsibility to make the world a less gender-binary place—we should all be anti-binary, right?—and then maybe in my next lifetime I will be able to be more psychologically nonbinary, which would be good.
Anyway, if you do not want these two long comments I have made, then I apologize for the intrusion, but I am appreciative of your article because I think about these things a lot and I rarely encounter them expressed in the way you have expressed them. I find your expression very helpful.
