This has become very common for either ported packages or single source packages for multiple operating systems. Think iTunes!
Hmm. I really don't wanna think iTunes. I detest the package... Strange, that, since I'm experimenting with MozDev's SongBird package -- an attempted fusion of iTunes and WinAmp.
However, in the spirit of RealPlaything, Windoze Media Plaything or WinAmp, or even notsoQuicktime; it might be rather nice to be able to change MIME types or file associations on the fly. It'd go a long way towards alleviating many of the file association frustrations I see in this forum -- including a few that I've experienced: I had to completely remove VLC from my system, including the registry, and reload it before it would acknowledge .mp4 as a valid VLC media file...
Cheers!