Sorry, I meant the `S` part, not the lack of 0 (i.e., to fix the import issue).  

I actually think we should allow 0-arg functions. They are useful. I have an [open issue](https://code.google.com/p/sympy/issues/detail?id=3885) for it. To see an example use-case, look up a project called ROFL by Jon Riehl and the corresponding talk from either SciPy or PyCon.