I agree with you if you've got 2x modern Spektrum, it's the easiest solution, that works well. But on a retro/single channel forum many (most?) of us don't have Spektrum sets. Special cases? I think not....
So how does it work if you've got 1x FrSky and 1x FlySky module? Or 1x Jeti and 1x FrSky module? Or 1x Futaba and 1x Orange DSMX module? It doesn't work! Some of us fly non-Spektrum radio's. They may be the most popular in the USA, but locally they don't account for 50% and are losing ground fast to FrSky and Jumper at the bottom end and Jeti at the mid to top end. Oh and I don't have a Spektrum (except the original DX6 that started all of the 2.4GHz off), but fly with Skyleader, Kraft, Graupner Variophon and Min-X with FrSky modules.
These radio combinations are hardly special cases, they are quite normal at our club where the pupil is encouraged to use their own transmitter and the instructor likes to use his own transmitter, not some cheap, Spektrum DX6 just for training.
Actually I do have a Spektrum DX5e set that I've taken the 2.4GHz module out of and used as a donor for an Arduino encoder that connects directly to the Spektrum module and is into flight testing, but that is for another time.