This seems like a good idea to me. Even if some random distro should fail to set that variable, a skin would just fall back to the defaults anyway.
Yep, and there's some people who use different desktop environments e.g. installing GNOME on Ubuntu (or using Ubuntu GNOME) - detecting the distro wouldn't tell MC which buttons to use for example as one could be using a different DE. That way the skin will always match the DE's buttons, if available.
I guess I'll see if I can rip the buttons from Ubuntu (Unity), Debian (GNOME) and Linux Mint (Cinnamon) at the max scale available within the next week. Then I'll rip Xfce and MATE's buttons.
EDIT: Found a list of what echo $XDG_CURRENT_DESKTOP can return - also found it should be used with echo $DESKTOP_SESSION you can gather which session is used (which is needed to detect KDE, apparently);
KDE:XDG_CURRENT_DESKTOP=
GDMSESSION=kde-plasma
Unity 3d:XDG_CURRENT_DESKTOP=Unity
GDMSESSION=ubuntu
Unity 2d:XDG_CURRENT_DESKTOP=Unity
GDMSESSION=ubuntu-2d
Gnome:XDG_CURRENT_DESKTOP=GNOME
GDMSESSION=gnome-shell
Gnome Classic:XDG_CURRENT_DESKTOP=GNOME
GDMSESSION=gnome-classic
Gnome Classic (no effects):XDG_CURRENT_DESKTOP=GNOME
GDMSESSION=gnome-fallback
Unity running with Mir:XDG_CURRENT_DESKTOP=Unity
GDMSESSION=ubuntu
Note MIR_SERVER_NAME is also set
MIR_SERVER_NAME=session-$NUMBER
Unity running without Mir:XDG_CURRENT_DESKTOP=Unity
GDMSESSION=ubuntu
XFCE:XDG_CURRENT_DESKTOP=XFCE
GDMSESSION=xfce