Will my tank eventually cycle ?

What they don't tell you is that having a protein skimmer removes compounds that decay into nitrogen and some phosphorus, without using Purigen. So does having macroalgae growing in-tank or in a refugium, a deep sand bed, microalgae on the back/side walls of the tank (which has the side benefit of feeding snails and herbivores), or even having photosynthetic corals in the tank.

Why resort to chemicals (Purigen included) when there's so many natural methods available to combat nitrate and phosphate buildup?

 

There's nothing really natural about keeping live fish/coral in a tank and replicating its natural environment. Unless you intend to have microorganisms as pets, sure, the nitrogen cycle works great.

 

Using ammonia-eating bacteria (natural) to convert ammonia to nitrite, nitrite-eating bacteria (natural) to convert nitrite into nitrate, then using algae/bacteria/corals (natural) to convert nitrate into nitrogen gas is insanely more natural than using a chemical resin (synthetic) to bind nitrogen until you use bleach (synthetic) to release it. This is just the tip of the iceberg; the nitrogen cycle provides more than just a method for removing ammonia from the tank.

You don't seem to be considering the possibility that the resin is binding things other than nitrogen and phosphorus compounds which you might want in your tank, like iron, which is needed for macro algae to grow. You also aren't considering the benefits of having bacteria in your tank for various things that you might actually want as pets to eat, like sand sifting stars or many filter feeders, or that algae is great food for the fish you have in the tank. You also don't consider that the algae is great food for various isopods, amphipods and copepods, which are great, natural food for a whole lot of fish.

Beyond everything I already mentioned, photosynthetic corals absolutely need some level of nitrate and phosphorus in the tank to create sugars. I wouldn't want to risk putting in something that destroys nitrate as well as this stuff does, especially when keeping non-SPS corals that require a detectable amount of nitrate. There's stories of tanks crashing anywhere from minutes to days after adding this stuff to the filtration; I can only wonder why.

Using Purigen, you remove the possibility of having a large amount of these various lifeforms in your tank, instead aiming for what I would describe as the saltwater equivalent of freshwater fishkeeping: a unnatural setting that is much farther removed from reality than it has to be.

 

Many of the things you state here are correct that nitrogen isn't the devil and that it plays a key role in many different levels of the food chain. You claim to have read stories about people having bad experiences with purigen and other nitrogen removers, but I'm claiming to have read plenty of positive stories resulting from these synthetics and how it has help stabilized their nitrogen levels; and it works great for me too. I'm sure there are many techniques and variations in using purigen as in the amount used, time length, placement... etc., in order to control how much dissolved compounds you think is best for your livestock. Experiment. I certainly don't think purigen is better than what's truly natural, but it is a great alternative if used carefully.