Besides the historical argument, there is also a certain theology presented in the observance of the Sunday of the Passion. In the liturgy of Passion Sunday, we - the church - are simultaneously the ones who pour accolades on the entering Jesus and the ones who demand his execution. The peculiar juxtaposition of celebration and crucifixion gets to the heart of what we believe about Jesus and the world. Yes Jesus is the Messiah, but not the Messiah of worldly conquest. Yes this creation is good, but it is broken by sin. Yes we have been made holy by Christ, and yet our wills are still incurvatus in se. Yes God's Kingdom has drawn near, but no it has not fully come into being.
The liturgy of the Sunday of the Passion will not allow any theology that dwells on triumphalism. It will not let us deceive ourselves about the sort of Messiah that Jesus is. To leave Palm Sunday without the Passion narrative is to leave with the impression that Jesus is the conquering Messiah, the Messiah of worldly power. The Passion narrative reminds us that we follow the crucified Messiah, the one who gained victory precisely in defeat.
(See also Mark Mummert's musings on why we read both the Synoptic Passion and John's Passion every year)