CYA significantly reduces UV degradation of FC. But, since more FC is needed when CYA is used, while the FC loss in percentage terms is lower, the chlorine depleltion in absolute terms is more when using CYA than without it.
This is based on the posts on this forum. The more CYA you use, the more chlorine you use, especially when there's lot's of direct sunlight. I'm not sure that's well understood... Maybe I've got this wrong, but that's what I think the technical data provided by this forum indicates.
I do understand why CYA is needed with a SWG. The SWG can only generate chlorine at certain rate. It might not be capable of generating chlorine as fast as needed to keep the FC high enough throughout the pool on a sunny day. So, the CYA "holds" chlorine in reserve for when needed.
However, an automatic chlorinator doesn't have this limitation--or at least it's not an issue for the amount of chlorine that needs dispensing on my 17000 gallon pool on the hottest, sunniest days. It could dose gallons of 12.5% chlorine per hour, if that were needed. Yes, the pump needs to be running, but it's programmed to always be running during the very sunny periods of the day.
The only real chore I need to do is to fill the Intellichem chlorine (and acid) tank every couple of weeks. I figure if I used CYA, I'd just have to do it more often, since I'd need more FC for the same sanitation. So, why should I use CYA?
Additionally, when if I need to SLAM the pool, it takes very little chlorine to reach shock level.
Finally, my dogs use the pool as their watering bowl. Without CYA, the FC needed for sanitation is less than municipal limits on drinking water. Not sure I can say that with CYA added.
I'd like to know if I'm missing something.