Also, you are servicing pools which is different than a homeowner who is maintaining their own pool on a daily or near-daily basis. For pool services that don't visit a pool that often, then having "insurance" against algae growth can be worthwhile, but there are all sorts of caveats as Jason mentioned.
As for metal sequestrants increasing phosphate levels, that is true for some sequenstrants including those containing HEDP.
So if you have a pool with primarily orthophosphate and not organic phosphates and no algae and there are not phosphates in the fill water (or you have a pool cover for minimal evaporation and refill) or other sources introducing more phosphate, and if the phosphate level isn't so high as to cause problems during removal (excessive cloudiness or filter clogging), then using a phosphate remover can work reasonably well. It would be insurance and is not necessary.
I used to have 3000+ ppb phosphates in my pool, probably from using HEDP metal sequestrants (I used all kinds of products when I first got my pool before learning how to properly maintain it) and from the 400 ppb phosphate in my fill water. The pool was easily maintained with the proper FC/CYA ratio, but if the FC ever got too low there was a noticeable increase in chlorine demand as algae could start to grow and use up chlorine. I would describe the pool as "reactive". Nevertheless, I was able to maintain the pool well until I neglected to add chlorine fast enough after opening one spring and turning on the solar panels (that were probably dirty inside so consumed chlorine faster). As described in the thread
It Can Happen to Anyone - Zero Chlorine, CYA-->Ammonia, the chlorine level got low, algae started to grow, chlorine level more rapidly got to zero, bacteria then grew and converted CYA into ammonia and partially broken down CYA chemicals creating a HUGE chlorine demand. I got threw it and the next year as described in the the thread
Orenda Technologies PR-10000 and CV-700 Products I got a free phosphate removal treatment (and enzyme treatment) which clouded the pool where I used a clarifier to more quickly clear it (my wife wanted to swim the next morning). Ever since then, my pool is no longer "reactive" and if the chlorine gets too low it doesn't show any noticeable increase in chlorine demand. Though I have 400 ppb phosphates in my fill water, my use of a cover means there is minimal evaporation and refill. Furthermore, the annual winter rains dilute the water to keep everything in check (mostly the salt level).
With all the caveats we don't recommend using a phosphate remover, but that does not mean that it doesn't work to significantly slow down algae growth, at least in certain situations.