One thing I noticed about installing multiple desktop environment on Ubuntu: they all have different default apps that are always installed. So when you have two desktop environments, you end up with 2 different terminals, 2 different file browsers, 2 different archive managers etc. You could uninstall apps that have the exact same purpose, but it would be nice if there was a formal way for DEs to recognize and ackno…
Not sure if it's what you want but applications can specify in their .desktop file that they should only be shown in a specific environment :
cat /usr/share/applications/xfce4-about.desktop
[...]
OnlyShowIn=XFCE;
Now many don't but I guess if they all did it would then be a problem for people who want to use applications from another DE.