There have been quite a few websites shut down and fined for hosting midi files of copyrighted music in the past few years. Mostly they were also hosting lots of ad banners to turn a buck off of other people's work. There have also been a few slimeballs that have been downloading vast quantities of midi files so they could sell their "50,000 midi files one one CD". So people who can make really good midi files stopped sharing them on the web and what you're left with is mediocre files by amateurs or ripped-off commercial files.

Think of midi files as player piano files in a digital format or as sheet music and you can get a better understanding of why copyrights apply to them the same as prerecorded music. In fact, the Library of Congress now allows you to submit original midi files for copyright as a recording.

The "why should poor me support copyrights when megastars are rich enough already" justifications only helps undermine the only protection that songwriters have. Most songwriters aren't wallowing in bucks - they are regular guys trying to sell the results of their talents and labor the same as everyone else and maybe get lucky. Copyrights are the only thing that keep a market for their work viable. Something about it being "just music" seems to justify the rip though, even to other musos.