First, the objects that cast the shadows don't have to look as though they have anything in common with the shadows themselves. Such is this piece at the Museum of Fine Arts:
Or in this advertisement:
Secondly, the meaning of the sculpture can change as the light source moves or changes. Such is this thesis project of an art student:
Time-lapse Shadow Verse
Both reasons are more mathematical than aesthetic, but the boundary is subtle. According to one definition, beauty is the sense of wonder, whereas art is the expression of that sense. Well, mathematics is all about wonder. Both of these examples are quite primitive from the mathematical point of view and an engineer with good geometrical skills could design something far more impressive. For some reason most engineers rarely give much thought to such projects. Could it be that engineers are less prone to respond to symbolism?
I actually like both pieces for their artistic value too. Shadows and the motion of the sun across the sky do convey a sense of the passing of time and the illusory nature of perception.
Any artists/mathematicians care to brainstorm a few ideas? For example, in the shadow sculpture above, it would not be too difficult to modify the pile of junk so that it casts for different meaningful shadows onto four different walls simultaneously, but that's just the beginning.