The harm would be that, by making the prophet's face into a ball of light, the artist would implicitly accept YOUR view of the historical Muhammed. I should imagine this was fairly obvious. Such a representation as good as calls for worship; it imposes on the viewer the idea that the man in question is a supernatural being, or a supernaturally visited one. In other words, you force us to lose any debate as to the truth of Muhammed's supposed reveleation, even before it started. And that is not good argument, nor good sense.