See http://bbs.archlinux.org/viewtopic.php?p=170060#170060 for more details.

Basically, applications requiring to link in libavcodec.so might not work if a call to avcodec_decode_video is made (and probably other functions) since this function requries the xvidcore shared libraries.

This can be demonstrated using the way ./configure scripts check for library compatibility. For example, the openCV application checks for avcodec_decode_video in libavcodec.so by writing a test app and attempting to compile it.

To reproduce, uninstall xvidcore, install ffmpeg and run:
echo 'int main(){ avcodec_decode_video(); return 0; }' | gcc -lavcodec -x c -o /dev/null -

You should see:
/usr/bin/ld: warning: libxvidcore.so.4, needed by /usr/lib/gcc/i686-pc-linux-gnu/4.0.3/../../../libavcodec.so, not found (try using -rpath or -rpath-link)

Now install xvidcore and rerun, there should be no output indicating a successful compile.