Although I'm a recovering sexp addict I must confess that I fail to see what makes the cons cell so important to those purer in their faith?

To me the homoiconity of Lisp is mainly about code-as-data, the exact nature of the data doesn't matter to me that much as long as it's a first class citizen and enclosed in nicely balanced parenthesis (though sadly here Janet seems to have fallen to the temptations of the curly braces, and thus, is indeed heresy).