Over the last 10-years, Schluter has introduced many new products...their training courses cover a bunch of them, and not only the use of Kerdi, but also uncoupling membranes, floor warming systems, tile profiles, hydronic heating, and many other products. If you want to stay up with what's new, you go back again. And, each time you do, you pick up little tricks and knowledge from not only the instructors, but from the other people attending the workshop. That John uses that as a slam, just shows he doesn't understand or appreciate talking to others that not only understand but also use the products in question and can point out their good and bad points. The stuff works. Is it the best, that's like asking is a Chevy or Ford a better vehicle, and one reason why both companies are still in business, they each have their good and bad points.
So, while most pros don't use the kits because the showers they tend to build require custom installations, doing something like a motel where all of the showers may be the same and it was designed to use the kit, they can save a bunch of time, and time is money for the motel owner.. If a kit works for you will depend. But, building a shower from the basic materials (Kerdi, Kerdiband, the Kerdi Drain, etc.) is fundamentally the same.
As you can tell, John doesn't like me, and disregards most anything I say. That is to his detriment. If you want a non-confrontational discussion, visit that other site I mentioned where you'll have access to faster, and more numerous inputs, from a site dedicated to tiling things.