MIDI is very limited. MIDI was set up 30 years ago as a communication interface, and by today's standards it's a poor one- you're limited to one note per millisecond. IIRC, you are also limited to 16 channels, so composing scores for an entire orchestra is out of the question.
To top it all off, it wasn't meant for music notation. Symbols like Accelerandos, Ritardandos are notably absent- changes to tempos are hardcoded. Many other symbols are absent as well. Sometimes notes need to be formatted in a special way (ie- for readability, or left/right hand on piano).
Anyone who has ever composed in Finale, Sibelius, etc and tried to export to midi will notice the limitations right away. Why, what's your beef with XML anyway?