Also, though not recommended on this forum (due to extra cost, more than anything else), there are other ways of dealing with higher CYA levels. One can use a weekly maintenance dose of PolyQuat 60 to be able to have somewhat lower FC levels at higher CYA levels. One could spend even more for a phosphate remover and have even lower FC levels with even higher CYA levels and not get algae. One could use a copper-based algaecide and prevent algae while simultaneously staining your plaster pool if the pH rises (or you use excess copper).

I generally don't like all or nothing rules when there are alternatives. As Carl says, if you measure your pool's water chemistry regularly, then you can stay on top of the CYA level. Also, in smaller pools with weekly backwashed sand filters and short swim seasons, the CYA level can be kept more in check such that even Trichlor could be used as a primary source of chlorine. It's all about knowledge and adjusting one's routine based on that knowledge.

Richard