What xmms2-et does
It sends a UDP packet for each song played with information about which output and xform plugins are used and what operating system xmms2 is running on. The information is sent anonymously and doesn't even contain the name of the song.
Why xmms2-et exists
By anonymously sending data about usage parameters, xmms2-et helps the developers to approximate how much testing different components in xmms2 get, which will help in making it a better music player.
If you have never had xmms2 installed before, this feature will be enabled automatically. If you already have xmms2 installed you can manually put the xmms2-et-launcher.sh script in
$XDG_CONFIG_HOME/xmms2/startup.d (or simply remove that directory to get it fresh from the skel).
If you, for some reason, don't want the phone home feature, you can disable it by removing the link within
$XDG_CONFIG_HOME/xmms2/startup.d or by not compiling "et" (scons EXCLUDE="et" or waf --without-optionals=et).
If you installed xmms2 from Debian packages, xmms2-et will not be installed by default - it is available in a separate xmms2-et package.
The result of xmms2-et can be seen at http://test.xmms2.org/