There's no added value to having multiple desktop environments, so almost no one would want to. A lot of applications use DE sensitive configurations and there's potential for conflicts as well as libraries incompatibility. Which can result on paradoxical and bizarre behavior from some graphical apps. It's odd that it happens but it's also not something devs plan or account for, so they aren't even considered bugs. You don't install multiple DEs at the same time unless you're purposefully trying to break something or you don't know better.
The only use case currently is choosing between a DE with X or one with Wayland. But even that one could fuck your system.
For example, opening cinnamon experimental Wayland makes all my flatpaks stop working until reboot. Why? I don't know, nobody knows. But if I keep using Wayland after reboot they work. If I change to regular cinnamon, they break again until reboot, when they get fixed as long as I keep using regular cinnamon. It just be like that.