Presumably there's something incompatible with how the Spotify client plays audio and how his system handles audio playback. His point still stands though - with MP3s he can use an alternative player, with Spotify he can't.
I use an ncurses Spotify client (I forget what) or a web browser to listen on my computer (normally I do it on the phone and airplay to the required speakers). Never had an issue.
My problem with Spotify isn't the DRM, since I use it mostly to play either background music or find different things to listen to--like I used to use satellite radio. The problem with Spotify is that the artists (for the most part) get so little while providing the content.
