As ironic as this may sound, less user choice means that there's less
room for error. Plus, it makes things conveniently easier for me and
possibly every other user of the software.
Now that I understand more about how GNU/Linux distributions, display
managers, and X sessions work, it makes sense to separate meta packages
based on environment used.
When you use a display manager, you're just starting X in a fancy way.
Since you can run any program you want on the X server, it is easy to
install multiple desktop environments on the same machine and switch
between them easily, provided your setup is adequately modular (which
it should be by default).