@Adrian Kingsley-Hughes
to answer one of your questions ("I’m left wondering: who are these codecs aimed at?")
Maybe for those 'few' people who just happen to live outside certain countries where software is usually patentable. ( As far as I know most of Europe, for example).
Regarding mp3 and libdvdcss I can recommend
http://en.wikipedia.org/wiki/Mp3#Lic..._patent_issues
and
http://en.wikipedia.org/wiki/Libdvdcss
respectively.
The patents around mp3 are (in my unprofessional opinion) a total mess.
And you might even (technically) infringe patents if you use purchased software from Microsoft.
Regarding libdvdcss according to wikipedia.org its legality has never been tried in court (unlike DeCSS), so the status is uncertain. Thus, if you want or need to use DVD playback, you either have to take the risk or pay someone to take it for you.