I’m a fan of dark themes myself, but one good reason to keep the white one would be to be able to reduce the screen brightness more than when using the dark one, especially in daylight, giving you more battery life.
Also, not all the apps are prepared for dark theme, where you could have unreadable black on dark gray text.
Taking the question at face value … for a start, if doing so requires a tutorial from @amarok then that might be a reason not to. It might suggest that that level of configurability is not ready for primetime.
But that then would lead to your next question: why is it not the default?
Did anyone have any issues with this change being reset on reboot? Both methods described set dark mode, but it reverts back to the default light theme when I reboot.
I had these package installed (withouth tweaks installed): Adwaita_pkg_for_dark_mode|690x225
Then just run gsettings set org.gnome.desktop.interface gtk-theme "Adwaita-dark"
Then it worked for me.