The Pax-Bellum( or any aggressive algae-based filtration in general) may not be absolutely necessary or justifiable as an investment in all cases. However, I will give two examples of tanks where, if used properly, it can make a difference: predator tanks and continuous feeding NPS(non photosynthetic corals). In both these cases, large amounts of nitrate, and especially phosphate, are continuously introduced by feeding.
Here's my experience:
I've been personally keeping an NPS tank for about three years, and I can say that could not win the battle with high phosphate. No matter what I've tried(Zeo, biopellets) I had to use large amounts of iron-based phosphate remover(1lbs for 100 gal tank changed weekly) in order to keep PO4 at an acceptable 0.2-0.3ppm(while NO3 ~0ppm). For me, this is unsustainable financially.
The first attempt was to re-balance nutrients by supplementing NO3 and adding NOPOX , but ALL my gorgonians immediately stopped opening, which I attributed as a side effect of excessive bacterial bloom. I retried again a few weeks later, after they eventually recovered, but got similar reaction.
Fast forward, I switched to a large LED lit DIY algae scrubber and increased flow + lighting on my 25 gal refugium, while starting NO3 + soluble iron dosing. Within 2 months, I was able to remove PO4 reactor, and PO4 remained stable at about 0.1 ppm. I clean half of the scrubber every 4-5 days. My gorgonians stay open at least 12h/day.
My conclusion is that, in high-nutrient environment, aggressive algae filtration can be used in confidence, with much lower risk than bacteria filtration. I can expect this kind of effects are also achievable with a Pax-Bellum reactor.