There are a number of problems with the "midi" attribute:
* The midi file is not discoverable, and is provided by a magic link wrapping the score image itself
* Users may click on the score expecting the image gallery viewer, but instead get a download they didn't want
* We now provide vorbis renderings with an embedded player, which solves the problem of letting the user hear the music.
* Editors are confused about which options to provide
I assume the reason we still have this feature is just legacy from before vorbis support was added. The user need (to hear the music) is much better satisfied by the vorbis feature.
The only reason to keep midi support would be
* For MW users who want to have score and audio rendering, but don't want to install TimedMediaHandler/vorbis libraries. I imagine such users don't exist. I don't believe we need to support it.
* For Wikipedia users who see a score rendering and really want the midi for some advanced usage. I imagine such use cases to be extremely low and such advanced users could extract the lilypond from source and generate the midi themselves.