I haven't looked closely at libmad yet so I don't know how useful the id3 support is but I assume it is as you say and I definitely agree. We intend to use id3 tags in Apollo [1]. Currently, Apollo is just a front-end to mpg123, but I'm looking forward to base Apollo primarily on libmad.
id3 tag support will be important for us (the Apollo developers) in the future, and we've discussed using id3lib [2], but I'd rather not depend on yet another library.
the id3 support is there, it just needs to made into a module. Not hard, just a little bit of time.
id3 is really easy. it boils down to a few static arrays and some bit walking. index + bit n => artist, index + bit m => title, etc. It basically sits just past the mp3 info and tags along.