I shan't presume to answer for Eliezer, but for myself, I think it is valuable that in the course of a day, I encounter many people whose experiences are similar to mine along all these gender-related axes, and with whom I can discuss these commonalities, and that, at the same time, I encounter many people whose experiences are drastically unlike mine, whose ways of thinking I will have to exert effort in order to understand. Thus, having a few genders seems to me to be well-optimized for fun.

(It is, of course, possible that I'm just putting a happy gloss on the circumstances in which I already happen to exist)