The idea that the colt was to be used to help Jesus get the word of God to more people is not the most common interpretation of this passage in Luke.  Instead, Biblical scholars generally believe that this is another way to emphasize that Jesus is the Messiah.  When Jesus has his disciples get the donkey, he is fulfilling various scriptural prophecies.

For example, Jesus is fulfilling the prophecy in Zechariah 9:9.  There, we are told that the king will come riding on a donkey.  Therefore, Jesus needs to enter Jerusalem at this most important time on a donkey.  He cannot just walk into the city since he realizes that this is the beginning of the events that will lead to his death.  In addition, it was generally the case in those days that animals that were to be used for some sacred purpose were not used for anything else.  This is why it is important that the colt/donkey had never been ridden before.

So, Jesus is not just obtaining the colt for practical purposes.  It has a symbolic importance as well.