Advantages of MusicXML
MIDI tracks hold MIDI notes and other MIDI data. A MIDI note in Cubase is only defined by its position, length, pitch and velocity. This is not enough to decide how the note will be displayed in a score. For a correct representation, Cubase also needs the following information:
Stem direction, beaming.
Expression marks (staccato, accent, ties and slurs).
Information about the instrument in the score.
Key and basic rhythm of the piece.
Grouping of notes, etc.
MusicXML can save a great part of this information. However, you have to adjust the scores with the tools available in the Score Editor.