It appears that the version of libmodplug inside gst-p-bad is heavily patched to make it usable, so we can't use the system version (which may or may not be usable).
From upstream gst-plugins-bad log:
Author: Hans de Goede <firstname.lastname@example.org>
Date: Sat Jan 24 18:13:39 2009 +0100
Build the modplug plugin against the modplug library and remove our copy
Always build the modplug plugin against the system modplug library
and remove our own copy. Using the system version has advantages
if security issues or other critical bugs are found in libmodplug
and our own copy wasn't really maintained anyway.
Also our copy only contained some patches to use GLib types and functions.
Fixes bug #568837.