While it may not be educationally necessary in a literate society, in a pre-literate society representational images can be very powerful. In illustrating some activity of Muhammed some representation of that activity can be a powerful educational tool for those who cannot read. Unfortunately, the power of abstraction comes from reading. The Qur'an certainly speaks of Muhammed, and thus creates pictures of him in words. Logical consistency would suggest the absurdity that these word pictures should also be forbidden. Abstract reasoning allows readers to understand that any representations of Muhammed or Jesus are not based on original photographs.
We cannot underestimate the power of images, symbols and myths as entities in their own right. These powers make no scientific sense at all, but that does not diminish their influence. Many Americans can be very upset when they see their flag being abused, yet to the strictly logical mind it is just a piece of cloth.
I think you're putting too much sway on *reading* the Qur'an. The Qur'an is often spoken aloud (and even memorised), so illiterate people can get just as much from the words as literate people can.