There's some discussion as to whether or not CYA levels that are higher provide any benefit. The answers seem to be:

1) If you have a SWCG, then the higher levels are mandatory (around 70) both for the correct functioning and to avoid voiding your warranty.

2) In extremely hot, muggy, sunny climes (like Louisiana) people find they just cannot keep chlorine levels consistent without higher CYA levels. Our La. Mod, aylad, finds she must always keep her pool at 70-80ppm of CYA or she cannot maintain her residual chlorine. Real experience trumps theory in this case.

In our extreme heat, here in NJ, where it's been in triple digits in both temperature and humidity (105 F and 100% humidity), despite my CYA of 50, I've been having to add LC every day.

3) It seems that if you cannot maintain a steady residual FC level, you must increase your CYA and target your residual level accordingly.

Hope this helps.