Yes. Explaining the subtle points of theology behind an association is possible only in long-desc cults. The question is "How many years will we have to wait for long-desc cults ?"
There actually is material out there that describes these cults in much greater detail than Cults of Glorantha does (or will ever be able to). Thunder Rebels spends a 100+ pages on just two cults: Orlanth and Ernalda. Storm Tribe is an entire book dedicated only to the other Orlanthi cults. Sometime next year, Moon Design will be releasing a one or two volume Unfinished Work which contains just about all of the non-Sartarite Orlanthi background material that Greg and I have written over the last ten years (including the definitive writeup on the Second Age Hendrikings - The Durengards Scroll). Lots of myths that have never been published and so on.
For the non-Orlanthi cults, the Glorious ReAscent of Yelm, the Fortunate Succession, and the Entekosiad give reams of information about the Dara Happans and other Pelorian cultures. Fortunate Succession gives a fair amount of detail about the Yelm cult in the Second Age. The Middle Sea Empire is another one of Greg's Unfinished Work that gives reams of material on the God Learners and the Zistorites.
For those folk who subscribe to Friends of Glorantha, you can read chapters from Greg's yet unfinished Harmast Saga (which will eventually be published) which gives a wonderful perspective on how Gloranthan magic and myth works.
Finally, if you can get your hands on a copy, Enclosure 1 & 2 have cult materials - including a RQish writeup for the Orlanthi cults that David and I did with much input from Greg.
Any rule system is going to have some problems modeling the diversity and flexibility of Gloranthan cults. If you treat stuff like Cults of Glorantha as guidelines or generalized statements that have lots of local variation to them, you are probably just fine.
Also, why are you people complaining so much about RQIII Cults of Glorantha ? It is my most used book and I don't know much of errors in it.
The old RQ3 Gods of Glorantha was a fine supplement in its time but it models Gloranthan cults in a very superficial way at best. We've learned a lot about Glorantha in the last twenty years and Greg has written an awful lot since then. I think it is a well edited, well written rules book. As a description of Gloranthan cults, I think it is mediocre at best.
From what I have seen, folk like Jeff and Aaron are doing a good job of balancing the tasks of having a simple and easy to understand "rulesy" writeup of these cults with the task of trying to provide enough information for folk to actually play in Greg Stafford's Glorantha. It is a difficult thing to do and my quibbles about various details are in no way meant to be a criticism of their effort.
Jeff