Like others said, there isn't anymore a reason to not use the native in_mp3 plugin:
It supports 24bit output (mad's goal)
It supports Replaygain (in_mpg123 goal)
It supports gapless playback (in_mpg123 goal)
The decoder has been compliant for years. (Reason why the others where started)
The others do not offer much more nowadays, and have features missing (Media library, some problems with streams like the Zinch said, ...)
About the ASIO plugin, i guess it is because using ASIO was the wrong approach to a problem that is small and doesn't happen always.
A kernel streaming plugin was developed (
http://www.stevemonks.com/ksplugin/ ) which should fulfill the ASIO plugin role for XP based machines.
I've also seen a thread about a WASAPI winamp output plugin (Vista and 7), but i'm not sure if it has had more development.
ASIO is still useful today, but not so much for an audio player.