When Moses was at the burning bush and he was using the excuse that he was a bad speaker Hashem told him to take Aron to be the speaker. Was one of the reasons for this decision not just about providing Moses with a voice to speak to Pharoah was it partly about giving the older brother a role to play in the action to free the Jews. Surley if it was simply about Moses being a bad speaker an all powerfull God could have cured his speech problems so there would have been no need to take Aron along.
There is never a clearcut explanation to the motives of Divine behavior, but your first comment is certainly reasonable (to give Aaron a role) Having Aaron in the picture emphasizes that Moshe is not a total outsider. The fact that a man with a serious speech defect can become a great leader and revered teacher is a great lesson about human values and would be lost if G-d miraculously healed him. The Torah teaches us to use our G-d given qualities and shortcomings to form our lives rather than to depend on miraculous intervention.