One of the reasons I feel I need the glossary is to avoid "state breaks" inside the story. For instance, I have the Scathae (singular: Scatha), a race of reptillian humanoids. They are just as widespread as Humans and everyone knows who they are. So, the first time a Scatha is encountered in the story, it would feel unnatural to have an explanation of what a Scatha looks like. I want to be able to say:
"Ilcas Northstar was tall, with scales of cobalt blue. He pronounced ridges above both eyes, but the right one was chipped - evidently a scar from a past batle."
This describes Ilcas Northstar as an individual. I do not want to have to say:
"The Scatha had a long snout, a body covered in hard scales and a yard-long tail."
Granted, I've not been published, but I have been an avid reader since the tender age of three, diving into fantasy (also granted) much later in life, yet thriving within it, and it has been my experience as both reader and writer in this genre that such things should never BE an issue.
What you're saying here is that that Scathae are a natural part of your world and wish to establish that fact for the reader, yet at the same time you feel that the reader would find it too alien to simply accept as you've dictated your populace does. This is where you find your own conflict, because your desires and your views do not coincide.
Example: I've got a race called the maeyenin (doesn't that look fun to pronounce? Say it however you wish), which has within it four different branches. All speak the same language though, and for this I would use Italics to let the reader know that the use of their own language isn't common knowledge, and should be taken special note of. What SHOULDN'T be taken special note of is the Dorian, the Doriancy, the Dorosai, the Dorai, any of them. These would be the top rulers in the Northern Solivian kingdoms, male, collective, female, and female sibling respectively. Through the context of the introduction of these terms is where the reader gets what the heck you're talking about. So there's no need to give any more information than you would give about a ball. Just because we assume the reader has never seen a ball doesn't mean we describe what a sphere is, how it interacts with gravity, what it's used for or why. We assume they understand terms like toss, bounces, throw, catch, etc, and explain that the ball participates in these things.
Similarly, a "dax" and a "sphyle" is a male and a female Scatha, respectively. But since everyone in the story knows this, I want just use the words without introduction and let the reader look them up.
While I certainly would not object to these terms being included for those who don't catch on immediately, or wish to have suspicions confirmed, I feel that it's the same situation.
Here, follow this.
Ilcas looked to the sphyle with dismay. "You speak out of turn, miss. I never said those things and I dislike that you should even imply that I would."
The Scatha female turned a glower upon him, scales flashing in the sunlight angrily, reflecting the glint now residing in her stubborn gaze. "Then it looks like you'll have to stew in your own displeasure then, for I know what I heard and you'll not have me convinced otherwise."
Ilcas would have said more, his ire inspired once more as he caught the tail end of a muttered curse about dax, and all men, in general. Well! "You think just because you're a sphyle you can speak to me that way?! You're no better than a human female!" At least, that's what he would have said had Jorel not caught his attention just then, his arms waving frantically over his head; the council must have returned their decision.
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For example. Right there, hopefully, any reader could pick up that a sphyle is at least related to sex in some way, as is dax, even if they don't know that it's specifically male and female, and not some title afforded by station according to sex.
And I do agree that the book shouldn't NEED the glossaries to be understood, but be supplemented and enhanced by them.