I have had over 3000 ppb phosphates in my pool and was able to recover from having 0 ppm FC in the past by shocking as described in ping's post or the Pool School (in fact, I've had bacteria convert CYA into ammonia creating a huge chlorine demand). There's one user on this forum with over 18,000 ppb phosphate and is able to manage the pool with chlorine alone. A service tech often doesn't have the time, but this site and its methods are for pool owners that don't just visit their pools once a week. Basically, if you get behind and algae is growing vigorously, you have to shock the pool with chlorine -- there's no way one can just superchlorinate (i.e. run at 100%) with the SWG cell and have any hope of keeping up, BUT once one kills off the algae one can have the SWG keep up if the FC is high enough for the CYA level.
We've had thousands of SWG pool owners come on the forum with pools in various states of phosphate level, algae, etc. and they are all able to have green algae killed off and the pools subsequently maintained even with high phosphate levels. Again, phosphate removers should be seen like algaecides -- like insurance -- so if someone wants to prevent problems that can occur if the FC level drops due to some spike in chlorine demand, then there are extra cost options such as phosphate removers or algaecides. It's up to the pool owner to decide which route they want to go. Most on this forum choose to simply maintain the FC for their CYA level, shock only for unusual situations, and not use other products at extra cost (except sometimes borates, mostly for other reasons such as extra pH buffering and sparkle to the water).
We've had thousands of SWG pool owners come on the forum with pools in various states of phosphate level, algae, etc. and they are all able to have green algae killed off and the pools subsequently maintained even with high phosphate levels. Again, phosphate removers should be seen like algaecides -- like insurance -- so if someone wants to prevent problems that can occur if the FC level drops due to some spike in chlorine demand, then there are extra cost options such as phosphate removers or algaecides. It's up to the pool owner to decide which route they want to go. Most on this forum choose to simply maintain the FC for their CYA level, shock only for unusual situations, and not use other products at extra cost (except sometimes borates, mostly for other reasons such as extra pH buffering and sparkle to the water).