First, neither tile nor grout is totally waterproof, so whatever you use in a shower must not be damaged by moisture. Typically, people would use cement board on the walls with a vapor barrier behind it since none of those components are waterproof, BUT, none of them are damaged by being wet. The vapor barrier prevents any moisture trapped in those materials from being in contact with the wood studs and structure. The same thing is true on the floor of your shower...there's a waterproof layer preventing water from getting to it.
So, it's not a long stretch to realize that if you do put drywall in a shower, if must be kept from ever getting wet or you'll have problems. This is exactly what the Kerdi membrane does...it is a very effective waterproofing layer. Just like the liner in your shower keeps the wooden subfloor from rotting out or ever getting wet, the membrane does it for not only the walls but the floor. So, if you built it properly - the drywall stays dry, the floor stays dry, and everything works out well.
As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. But, after installation and a flood test, the entire surface is covered with tile, so that layer remains well protected, and is easily able to do its job.
Some related concepts: why can you use wood underneath your roofing shingles? Because when properly installed, the roof doesn't leak. Why can you use drywall on the inside walls next to your windows? Because a properly installed window doesn't leak. Sure, you can break the window or have wind blow a shingle off, but that's hardly likely to happen inside of a properly built shower covered in tile!
Schluter has ben using Kerdi over drywall for decades, and since the certification doesn't last forever, has been getting it retested to reverify it still works to stay in compliance bi-annually. The ICC (International Code Council) test report ESR-2467 covers the certification for both Ditra and Kerdi and is only valid if you follow the manufacturer's installation instructions. While other methods might work over drywall, to my knowledge, none have the history or the certification, or test data to show that they work properly in that environment. So, do not take Schluter's experience with Kerdi make you think that another system is certified, tested, and warranted in other than their tested, approved substrates.
FWIW, that report is valid over the following surfaces in a shower (from section 2.0):
"Kerdi is intended for use on floors over concrete, mortar and tile backerboard substrates. Kerdi is also used as shower lining, as required in IRC Section P2709.2 and IPC Section 417.5.2, over concrete, mortar, tile backerboard, expanded polystyrene foam board, prefabricated polystyrene shower trays, curb and ramp, gypsum board and masonry substrates."
So, you can choose what board you want in your shower when using Kerdi. Schluter's recommendation is drywall since it is available in larger sheets so you have fewer seams, accepts thinset well, and allows excess moisture from the thinset to be purged while maintaining a suitable environment for the thinset to cure and attain its best strength.
Kerdi will work equally well with any of the approved substrates - choose the one you want, install it per the manufacturer's instructions and you'll have a long-lasting, problem-free shower. Don't let any nay-sayers force you into any other extra steps by making you feel guilty...IF you do it right, it will not leak. IF it does, fix it before you put up your tile. Based on the certification, doing it otherwise could void the warranty and the certification of the Building Codes.