As I was looking at implementing preset profiles for the ffmpeg mpeg4 encoder I was pointed to some code which mentioned that the profile information was taken from xvid. While the xvid decoder and encoder defaults to 'Unrestricted' format which essentially creates a 'xvid' file they also support normal MPEG4 (and probably DivX) files too. 

I will test various things against the decoder so we can see how its caps should be expanded.

As for the encoder. If we set stuff like the profile property we can get it to output stuff like MPEG4 part 2 Simple Profile compliant files. So the encoder should also get caps for stuff like video/mpeg,mpegversion=4

Tim also mentioned on IRC that the plugin code is probably causing this plugin to be rather unstable. Which is a shame because in some respects this does seem like a better featured decoder/encoder than the ffmpeg one, at least they did the job of letting you set the profiles here :)