Its a huge deal. If X desktop is the default, it shows that the distro developers and maintainers usually test and optimize more and better for that specific DE so your experience with the default DE will always be more stable and polished than non-official ones. Extra GUI tools that the distro makes usually are also better tailored to the default distro. Like Manjaro and all of their locale, kernel and other packages that are integrated inside the KDE settings. Or popOS and all of their utilities being integrated into Cosmic. Etc etc. More money and dev time is invested into the default DE.