Because this way all the accesses to ExprEvalEvaluator go through a instance method of the DDLElementRegistry which is already percolated to the deepest level it is needed (the various DDXMLElements or whatever they are called). This allows me to remove all the references to `ExprEvalEvaluator::instance` apart from this one and the making it just a property of DDLElementRegistry will do the trick of getting rid of the ExprEvalEvaluator singleton.